diff --git a/RELEASENOTES.md b/RELEASENOTES.md
index e2713f16bf..b1e054a0bf 100644
--- a/RELEASENOTES.md
+++ b/RELEASENOTES.md
@@ -107,6 +107,12 @@
 * Fix the start of audio getting truncated when transitioning to a new
   item in a playlist of opus streams.
 * Fix detection of Dolby Atmos in HLS to match the HLS authoring specification.
+* Fix FLAC extension build
+  ([#6601](https://github.com/google/ExoPlayer/issues/6601).
+* Update the ffmpeg, flac and opus extension build instructions to use NDK r20.
+* Update the ffmpeg extension to release 4.2. It is necessary to rebuild the
+  native part of the extension after this change, following the instructions in
+  the extension's readme.
 
 ### 2.10.6 (2019-10-17) ###
 
diff --git a/extensions/ffmpeg/README.md b/extensions/ffmpeg/README.md
index cc9ea2a8c7..f8120ed11b 100644
--- a/extensions/ffmpeg/README.md
+++ b/extensions/ffmpeg/README.md
@@ -29,7 +29,7 @@ FFMPEG_EXT_PATH="$(pwd)/extensions/ffmpeg/src/main/jni"
 ```
 
 * Download the [Android NDK][] and set its location in a shell variable.
-  Only versions up to NDK 15c are supported currently.
+  This build configuration has been tested on NDK r20.
 
 ```
 NDK_PATH="<path to Android NDK>"
@@ -50,7 +50,7 @@ ENABLED_DECODERS=(vorbis opus flac)
 ```
 
 * Fetch and build FFmpeg. For example, executing script `build_ffmpeg.sh` will
-  fetch and build FFmpeg release 4.0 for armeabi-v7a, arm64-v8a and x86:
+  fetch and build FFmpeg release 4.2 for armeabi-v7a, arm64-v8a and x86:
 
 ```
 cd "${FFMPEG_EXT_PATH}" && \
diff --git a/extensions/ffmpeg/src/main/java/com/google/android/exoplayer2/ext/ffmpeg/FfmpegLibrary.java b/extensions/ffmpeg/src/main/java/com/google/android/exoplayer2/ext/ffmpeg/FfmpegLibrary.java
index 58109c1666..5b816b8c20 100644
--- a/extensions/ffmpeg/src/main/java/com/google/android/exoplayer2/ext/ffmpeg/FfmpegLibrary.java
+++ b/extensions/ffmpeg/src/main/java/com/google/android/exoplayer2/ext/ffmpeg/FfmpegLibrary.java
@@ -34,7 +34,7 @@ public final class FfmpegLibrary {
   private static final String TAG = "FfmpegLibrary";
 
   private static final LibraryLoader LOADER =
-      new LibraryLoader("avutil", "avresample", "avcodec", "ffmpeg");
+      new LibraryLoader("avutil", "avresample", "swresample", "avcodec", "ffmpeg");
 
   private FfmpegLibrary() {}
 
diff --git a/extensions/ffmpeg/src/main/jni/Android.mk b/extensions/ffmpeg/src/main/jni/Android.mk
index 046f90a5b2..22a4edcdae 100644
--- a/extensions/ffmpeg/src/main/jni/Android.mk
+++ b/extensions/ffmpeg/src/main/jni/Android.mk
@@ -22,12 +22,17 @@ LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
 include $(PREBUILT_SHARED_LIBRARY)
 
 include $(CLEAR_VARS)
-LOCAL_MODULE    := libavutil
+LOCAL_MODULE    := libavresample
 LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
 include $(PREBUILT_SHARED_LIBRARY)
 
 include $(CLEAR_VARS)
-LOCAL_MODULE    := libavresample
+LOCAL_MODULE    := libswresample
+LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
+include $(PREBUILT_SHARED_LIBRARY)
+
+include $(CLEAR_VARS)
+LOCAL_MODULE    := libavutil
 LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
 include $(PREBUILT_SHARED_LIBRARY)
 
@@ -35,6 +40,6 @@ include $(CLEAR_VARS)
 LOCAL_MODULE := ffmpeg
 LOCAL_SRC_FILES := ffmpeg_jni.cc
 LOCAL_C_INCLUDES := ffmpeg
-LOCAL_SHARED_LIBRARIES := libavcodec libavresample libavutil
+LOCAL_SHARED_LIBRARIES := libavcodec libavresample libswresample libavutil
 LOCAL_LDLIBS := -Lffmpeg/android-libs/$(TARGET_ARCH_ABI) -llog
 include $(BUILD_SHARED_LIBRARY)
diff --git a/extensions/ffmpeg/src/main/jni/Application.mk b/extensions/ffmpeg/src/main/jni/Application.mk
index 59bf5f8f87..7d6f732548 100644
--- a/extensions/ffmpeg/src/main/jni/Application.mk
+++ b/extensions/ffmpeg/src/main/jni/Application.mk
@@ -15,6 +15,6 @@
 #
 
 APP_OPTIM := release
-APP_STL := gnustl_static
+APP_STL := c++_static
 APP_CPPFLAGS := -frtti
 APP_PLATFORM := android-9
diff --git a/extensions/ffmpeg/src/main/jni/build_ffmpeg.sh b/extensions/ffmpeg/src/main/jni/build_ffmpeg.sh
index 358d2a2b8f..a76fa0e589 100755
--- a/extensions/ffmpeg/src/main/jni/build_ffmpeg.sh
+++ b/extensions/ffmpeg/src/main/jni/build_ffmpeg.sh
@@ -32,9 +32,10 @@ COMMON_OPTIONS="
     --disable-postproc
     --disable-avfilter
     --disable-symver
-    --disable-swresample
     --enable-avresample
+    --enable-swresample
     "
+TOOLCHAIN_PREFIX="${NDK_PATH}/toolchains/llvm/prebuilt/${HOST_PLATFORM}/bin"
 for decoder in "${ENABLED_DECODERS[@]}"
 do
     COMMON_OPTIONS="${COMMON_OPTIONS} --enable-decoder=${decoder}"
@@ -42,13 +43,14 @@ done
 cd "${FFMPEG_EXT_PATH}"
 (git -C ffmpeg pull || git clone git://source.ffmpeg.org/ffmpeg ffmpeg)
 cd ffmpeg
-git checkout release/4.0
+git checkout release/4.2
 ./configure \
     --libdir=android-libs/armeabi-v7a \
     --arch=arm \
     --cpu=armv7-a \
-    --cross-prefix="${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/${HOST_PLATFORM}/bin/arm-linux-androideabi-" \
-    --sysroot="${NDK_PATH}/platforms/android-9/arch-arm/" \
+    --cross-prefix="${TOOLCHAIN_PREFIX}/armv7a-linux-androideabi16-" \
+    --nm="${TOOLCHAIN_PREFIX}/arm-linux-androideabi-nm" \
+    --strip="${TOOLCHAIN_PREFIX}/arm-linux-androideabi-strip" \
     --extra-cflags="-march=armv7-a -mfloat-abi=softfp" \
     --extra-ldflags="-Wl,--fix-cortex-a8" \
     --extra-ldexeflags=-pie \
@@ -60,8 +62,9 @@ make clean
     --libdir=android-libs/arm64-v8a \
     --arch=aarch64 \
     --cpu=armv8-a \
-    --cross-prefix="${NDK_PATH}/toolchains/aarch64-linux-android-4.9/prebuilt/${HOST_PLATFORM}/bin/aarch64-linux-android-" \
-    --sysroot="${NDK_PATH}/platforms/android-21/arch-arm64/" \
+    --cross-prefix="${TOOLCHAIN_PREFIX}/aarch64-linux-android21-" \
+    --nm="${TOOLCHAIN_PREFIX}/aarch64-linux-android-nm" \
+    --strip="${TOOLCHAIN_PREFIX}/aarch64-linux-android-strip" \
     --extra-ldexeflags=-pie \
     ${COMMON_OPTIONS}
 make -j4
@@ -71,8 +74,9 @@ make clean
     --libdir=android-libs/x86 \
     --arch=x86 \
     --cpu=i686 \
-    --cross-prefix="${NDK_PATH}/toolchains/x86-4.9/prebuilt/${HOST_PLATFORM}/bin/i686-linux-android-" \
-    --sysroot="${NDK_PATH}/platforms/android-9/arch-x86/" \
+    --cross-prefix="${TOOLCHAIN_PREFIX}/i686-linux-android16-" \
+    --nm="${TOOLCHAIN_PREFIX}/i686-linux-android-nm" \
+    --strip="${TOOLCHAIN_PREFIX}/i686-linux-android-strip" \
     --extra-ldexeflags=-pie \
     --disable-asm \
     ${COMMON_OPTIONS}
diff --git a/extensions/flac/README.md b/extensions/flac/README.md
index b4b0dae002..e534f9b2ac 100644
--- a/extensions/flac/README.md
+++ b/extensions/flac/README.md
@@ -28,8 +28,8 @@ EXOPLAYER_ROOT="$(pwd)"
 FLAC_EXT_PATH="${EXOPLAYER_ROOT}/extensions/flac/src/main"
 ```
 
-* Download the [Android NDK][] (version <= 17c) and set its location in an
-  environment variable:
+* Download the [Android NDK][] and set its location in an environment variable.
+  This build configuration has been tested on NDK r20.
 
 ```
 NDK_PATH="<path to Android NDK>"
diff --git a/extensions/flac/src/main/jni/Application.mk b/extensions/flac/src/main/jni/Application.mk
index eba20352f4..e33070e121 100644
--- a/extensions/flac/src/main/jni/Application.mk
+++ b/extensions/flac/src/main/jni/Application.mk
@@ -15,6 +15,6 @@
 #
 
 APP_OPTIM := release
-APP_STL := gnustl_static
+APP_STL := c++_static
 APP_CPPFLAGS := -frtti
 APP_PLATFORM := android-14
diff --git a/extensions/opus/README.md b/extensions/opus/README.md
index af44e84b04..ce88f0ef7d 100644
--- a/extensions/opus/README.md
+++ b/extensions/opus/README.md
@@ -28,7 +28,8 @@ EXOPLAYER_ROOT="$(pwd)"
 OPUS_EXT_PATH="${EXOPLAYER_ROOT}/extensions/opus/src/main"
 ```
 
-* Download the [Android NDK][] and set its location in an environment variable:
+* Download the [Android NDK][] and set its location in an environment variable.
+  This build configuration has been tested on NDK r20.
 
 ```
 NDK_PATH="<path to Android NDK>"
diff --git a/extensions/opus/src/main/jni/Application.mk b/extensions/opus/src/main/jni/Application.mk
index 59bf5f8f87..7d6f732548 100644
--- a/extensions/opus/src/main/jni/Application.mk
+++ b/extensions/opus/src/main/jni/Application.mk
@@ -15,6 +15,6 @@
 #
 
 APP_OPTIM := release
-APP_STL := gnustl_static
+APP_STL := c++_static
 APP_CPPFLAGS := -frtti
 APP_PLATFORM := android-9