This appears to be a defect with the OBS Media Player. The defect seems to occur only when using the Media Player with a local MP3 music file, and only if the MP3 file has an image attached. I have not seen it in any other audio file format.
Typically, you will load a local MP3 file to OBS as a “Media Source” and enable monitoring to a Windows output device. I understand that this uses the “FFmpeg” media player.
If the MP3 song does NOT have a cover image (MP3-Tag ID3 frame 'PIC' or 'APIC') everything works fine as intended.
Any MP3 song with a cover image causes a set of issues:
The song usually starts and continues until one of the following:
If the scene is briefly changed. After returning to the scene with the song, the song will no longer be monitored.
If the end of the song is reached. If the song is supposed to loop, it will loop, but it will no longer be monitored.
If you interfere with the source via the media control, i.e. by stopping the song or using the media slider, the song will no longer be monitored.
If monitoring the song ended unintentionally due to any of the above issues, it can be manually restarted via “Advanced Audio Setting”, stop monitoring and start monitoring again.
I also found two related issues I had reported in the past but I can still reproduce them in the current OBS release 29.2.1 (64-bit)
1. Monitoring a looping song containing a cover image will stop monitoring at the first loop but monitoring will restart by itself after a single unmonitored loop.
2. Any MP3 song with a cover image will cause the above problems with monitoring. If you later replace the song with another song (via Source properties – select another local file without an image), then the old image stays on the scene. The new and different song will work fine but now the old image is stuck with the Source on the scene until you remove the source altogether.