I've been suffering this problem for years, and none of the solutions proposed here on the forums worked. I had lost all hope. However, recently I found a Japanese post (https://qiita.com/kbone/items/20779267094a564f2798) about this problem, which offers a working solution. TLDR:
- Compile Simutrans with mixer_sdl2
- Install fluidsynth (I've tried with timidity++, with no luck)
- Install a soundfont
- Open Simutrans with SDL_SOUNDFONTS=/path/to/soundfont variable set
I get some ALSA complains, but nothing that prevents the music from playing. The downside is that Simutrans will take significantly more time to startup. But I finally got to listen the Simutrans music :D
P.D.: I've updated my Arch Linux packages to compile against sd2_mixer, change the timidity dependency to fluidsynth, and display this information after installing the game.