--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -3683,16 +3683,19 @@ MediaDecoderStateMachine::LoopingChanged
     mReader->SetSeamlessLoopingEnabled(mLooping);
   }
 }
 
 TimeUnit
 MediaDecoderStateMachine::AudioEndTime() const
 {
   MOZ_ASSERT(OnTaskQueue());
+  if (mReader->GetDuration().IsValid()) {
+    return mReader->GetDuration();
+  }
   if (mMediaSink->IsStarted()) {
     return mMediaSink->GetEndTime(TrackInfo::kAudioTrack);
   }
   return GetMediaTime();
 }
 
 TimeUnit
 MediaDecoderStateMachine::VideoEndTime() const

--- a/dom/media/ReaderProxy.h
+++ b/dom/media/ReaderProxy.h
@@ -83,16 +83,20 @@ public:
 
   void SetCanonicalDuration(
     AbstractCanonical<media::NullableTimeUnit>* aCanonical);
 
   void SetSeamlessLoopingEnabled(bool aEnabled);
 
   void AdjustByLooping(media::TimeUnit& aTime);
 
+  // Return the duration of the media track. The returned value is valid only
+  // when the whole media has been decoded.
+  media::TimeUnit GetDuration() { return mAudioDuration; };
+
 private:
   ~ReaderProxy();
   RefPtr<MetadataPromise> OnMetadataRead(MetadataHolder&& aMetadata);
   RefPtr<MetadataPromise> OnMetadataNotRead(const MediaResult& aError);
   void UpdateDuration();
   RefPtr<SeekPromise> SeekInternal(const SeekTarget& aTarget);
 
   RefPtr<ReaderProxy::AudioDataPromise> OnAudioDataRequestCompleted(