I think I've done it, just don't know how correct my solution is
I've added snd_seq_drain_output() after snd_seq_event_output() in thread function so the events are processed more "real time" as I understand it. I've also moved queue creation to the beginning of thread function (because stop function deletes the queue).
Then the stop function calls: sets a bool to break the thread loop. snd_seq_free_queue() snd_seq_drop_output()
This seems to work. Any comments?
One problem I got with it though is that sometimes when I stop one music and play another one some note(s) from the old one are played indefinitely during the new music. I suppose some 'clear' event should be sent to remove it. I've tried MIDI_CTL_ALL_SOUNDS_OFF control one (as pmidi does on exit) but no luck.