Because Thorium is Chromium based and does not support mp3 and mp4 out of the box because they use proprietary codecs. We enable building Qt with the proprietary codecs support but it requires our own Qt builds. So if you are using a system Qt on Linux it may not play mp3 and mp4.
Correction:
Actually Thorium does support the proprietary codecs. They had a bug:
https://github.com/edrlab/thorium-reader/issues/1612
That was fixed 10 days ago in a recent continous test build that can be grabbed from the Thorium github site.