Custom soundfonts?

[HypnoHypno]

I'd like to use Microsoft GS Wavetable Synth soundfont for the audio, but I can't find out how the soundfont files are structured for me to do that.

[cifkao]

This is answered in the FAQ:

How can I use custom samples?

The player supports "SoundFonts" in a special format designed by Magenta. If you want to use a .sf2 file, it will not work out of the box, but it is possible to convert it with some effort. See this discussion thread and especially this answer, which proposes a conversion script.