No sound after upgrade from Debian Stretch to Buster? See your soundcard in /proc/asound/cards and alsamixer, but not in KMix? You know what your problem is, don't you?
Good luck trying to debug the situation. Been there, hopeless. Just doesn't make sense. You need to either know the answer or have had been through enough pain with pulseaudio in the past to get that hunch.
Here's your one-line fix:
sudo apt remove timidity
I hate you, Lennart! I hate you.