Found the problem in the wildmidi code, midi works now in the CIA. (just redownload the file I linked)
Further updates:
Disabled pitch changes (playback speed up/down) for BGM (background music) on the old3ds because the CPU can’t handle this (I run the BGM code on the 2nd CPU core and you only get 30% CPU time on this core because the operating system runs there. So the CPU time is very limited and pitch changing (which postprocesses the bgm buffer and for speedup even has to decode more samples) exceeds it and the music starts to crackle). Advantage of this solution: BGM does not influence the frame rate at all.
I’m sometimes getting a “data abort” crash when shutting down the Player. No idea why this happens, yet.