No, I'll give Kodi a try, thanks!
I didn't respond to the previous message because the conversation seemed unable to convey what I was trying to describe.
As you indicate, this would not be achieved by playing an mp3 file directly within Quest2 device (unless it replaces the video audio, which isn't what I'm looking to do). The Quest2 would need a 2nd audio chipset & output (primary would be for the video's normal audio to the viewer's headphones, the 2nd being the mp3 audio to the estim device). Even if this was possible using a USB-C audio interface, I'm not sure the Quest2 could support it properly. Hence the simpler solution seemed to be sync'ing to an external tablet's mp3 player (with its own audio interface, headphone socket, etc.).