It turns out SDL added a built in fluidsynth backend about a year ago, so now you can dump that flat eawpats sound for real soundfonts! Assuming your libsdl_mixer is new enough you can just use a line like this:
export SDL_SOUNDFONTS="Fluid-R3.sf2:/path/to/fallback.sf2"; odalaunch
Yep, that's all there is to it. You probably want to try a few different soundfonts since they don't always work perfect (weird instrument mismatches, clipping at high volume).
