Which music driver does this actually compile the game with? There is no "aplaymidi" driver in the official version, and if this uses the extmidi driver to call an external program, then the playback issues are not in OpenTTD code.

If you read the gentoo package itself they do this:

+	else
+		myopts+=(
+			$(usex aplaymidi '--with-midi=/usr/bin/aplaymidi' '')

https://gitweb.gentoo.org/repo/gentoo.git/tree/games-simulation/openttd/openttd-1.9.1.ebuild

Okay that implies it builds with the extmidi driver, and extmidi simply calls an external program to do all the playback. Have you tried running aplaymidi from the commandline to play the MIDI files? It will probably fail in the same way.

Yes,
When doing something like:
aplaymidi --port 128:0 gm_tt00.gm

and then do control+c to kill the command...

The sounds fails in the similar way.
and I have to press the panic button in Qsynth

So not sure to where belongs this issue.

Thanks anyways

I'm closing this as the issue is not in OpenTTD code.

A possible workaround for the missing note-offs on skip track could be if aplaymidi can be instructed to send various reset messages on startup (ideally a GM init message F0 7E 7F 09 01 F7) which would only leave the issue of hanging notes on exit. That could be worked around by starting OpenTTD via a shell script that performs a similar synth reset after OpenTTD exits.

The extmidi driver in OpenTTD is designed for softsynths that naturally end all sound and implicitly reset on exit/startup, not piping to external synths/hardware synths, it would require a major reworking of it to make that work. That effort would probably be better to put into writing a dedicated ALSA-MIDI driver for OpenTTD to natively output to a hardware port.