Ok, I think I figured out the difference between getting a dummy output or having the output there but not working for the second user.
If user 1 starts playing something before user 2 logs in an starts their client, then user 2 can't see (or select) the output in Sound Preferences (which usually would give them a dummy output if they have no other sound devices, right?). If both users have their pulseaudio going before one starts to play something, then user 2 can see the output device, but their sound applications can't play anthing.
Ok, I think I figured out the difference between getting a dummy output or having the output there but not working for the second user.
If user 1 starts playing something before user 2 logs in an starts their client, then user 2 can't see (or select) the output in Sound Preferences (which usually would give them a dummy output if they have no other sound devices, right?). If both users have their pulseaudio going before one starts to play something, then user 2 can see the output device, but their sound applications can't play anthing.
I'll attach the logs.