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The same with me - latest debian testing/sid. All soundcards were visible and functional (headset for example) in PulseAudio except the internal one. 'aplay -l' listed all cards including the internal but when I've tried to use it, I encountered the "audio open error: Device or resource busy". After uninstalling 'timidity' everything went OK.
Problem is fully solved for me now. The latest TiMidity update claims to fix the bug, and while I do not understand how it's been fixed, I have installed the new version, started TiMidity and played some MIDI files with it. Both PulseAudio and TiMidity are working fine together now.
Many thanks for that. Got some updates this night through unattended-upgrades.
This destroyed a part of my morning searching why I suddenly had no sound, I should come here more often.