Hi, I think the problem is related to the fact that, on a Debian system running pulseaudio, timidity-daemon still tries to add the "timidity" user to the "audio" group. It shouldn't. On such systems (at least those where the pulseaudio daemon is not being run system-wide) the "audio" group should be empty. This is well explained here: https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/PerfectSetup/ section «Should users be in the "audio" group?».
Emptying the "audio" group and rebooting the box fixed the issue. (To whoever might read: only do this after taking precautions) Hope this helps. Thanks for your work! Cheers, Gabriele :-)