Hi, glad to see you join here, and as you've contributed to Xonotic you are probably already aware of licensing issues and the like.
Our page explaining STK licensing requirements can be found here: https://supertuxkart.net/Licensing
With that out of the way, there's also this page: https://supertuxkart.net/Music_and_SFX_Guidelines
However as we already have two people working on the music, I'd say better sound FX for many parts of the game would be very welcome. Of course if you want to work on music it's possible, but there could be duplication of efforts issues requiring more communication with the musicians already around.
As you can see the page is very sparse on sound effects requirements, but just as a starting point there's https://sourceforge.net/p/supertuxkart/ ... x-lossless
with lossless versions of the sound effects found in the data folder of the game. It would especially be nice having better (and perhaps several?) kart colliding sounds and engine noise(s) since those are some of the effects heard most often, and also complained about most often. Some of that might be because they just get repetitive, and personally I don't find the sounds themselves that bad but the repetition make them a bit tiring.
There's other sound effects too, including track specific ones. I'd also say that more track SFX would be nice, so if you find something in the tracks that could become more alive by adding a sound effect, it would be nice to have a list of suggestions, even better if you find/create those.
If you have questions, feel free to ask here or on IRC. Obviously the latter is a bit restricted due to different people being available at different times, but with some patience it can work well too for communicating.