What elmer said. I believe it's the VGM format used for lots different systems (sms, gb, genesis, etc). The HES is a VGM player. The source is open source if anyone wants to write a non-cooked version of the music player, but VGM method is a fastest and easiest way to get the sound playing on the real system (people are writing the music just for music development, and not for game or such needs). Though not all the PCE ones are 60hz, IIRC. Some use the timer interrupt (I think there's a mode in the deflemask module itself for handling/setting tempo stuffs).
If you want the modules (to export to HES files), you'll have to PM the authors (which are the youtube account holders). Ruko was kind and gave me a few when I asked, a number of months back.
I was going to make a music disk volumes for some of these, but the VGM part needs a layer of compression to get the size down. I hadn't finished writing a VGM parser to do it yet.
