In addition, hex-a-hop upstream (but not Debian) has some non-free
Thanks! I'm surprised about Hex-a-hop but they marked the files very nicely. Also at least one file is based on a now CC0 sound. Freesound introduced alternatives to CCSampling+ a few years ago and some users are migrating with a long delay.
Do any musicians from there release source material for their tracks?
It is not common but some LMMS , midi  and milkytracker  sources are available, when using the format/sotware names as search terms. There might be even more.
Is there a definition of "source" by Debian available? When taking the GPL's definition "the preferred form of the work for making modifications to it" one could argue that a final rendered audio file can be easier to make changes to than acquiring and setting up the same software (and possibly sample libraries?).
Also, is there a clear official statement by Debian that data is required to be available in source form anywhere? I was trying to find this out for slides about free art in free games  two months ago but failed.