Just wanted to add that since posting the above, I have tried
"Background Player 2.25" by Vanel Remi. This sample player appears to
work correctly with AVR-files also under newer MiNT. And it seems
like Janez Valant (Hi SWE!) could reproduce the exactly same problem
with GEMJing under newer kernels, and he also experienced that using
background player would work alright. At least someone is able to
reproduce problems I am seeing for a change ;)
SWE also pointed out that it seems like the Falcon sound matrix is
being messed up (reset?) when the XBIOS call issued, so that the
sound is not redirected correctly from DMA.. ...this is too weird
stuff for me :/