playing midi files via timidity now works without stuttering etc, just uncommented the recommended settings for slow cpu's in /etc/timidity/timidity.cfg.
But it seems the pi only handles one sound source at one time.
Starting openttd without sound:
./openttd -s null -> music works, sound obviously not
I read somewhere that maybe some ALSA compatibility mode could help, I keep trying, I love this game and music, would hate to make a trade-off between sound and music