Comment # 3 on bug 1219070 from Eric van Blokland

I can confirm this issue.

It appears this is related to the audio decoder, maybe missing codecs?

When playing a mp4 video file with an opus audio track, there is no sound. The
video tag shows a grayed out speaker icon, as if the video has no audio track.
The video track was h264 and playback was fine.

Youtube gives the following debug information when failing playback:

"vemsg.DECODER_ERROR_NOT_SUPPORTED_ audio decoder initialization failed"