Hi, For the record, I had the exact same problem on a computer running buster that I don't use very often. For sure, it was working fine even with timidity installed a few months ago. Many thanks to Andrei for the 'lsof | grep /dev/snd' command that pointed us in the right direction! Debugging these sound issues that appear spontaneously on a previously-working setup is not easy, especially now that PulseAudio is required everywhere.
Regards -- Florent