Help with Custom SFX

Hi.
So I've been wondering about this for a bit. I got custom music working, so I decided I wanted to change the race finish theme, however I realized that was a sound effect. I then realized I have literally no clue how to do that. I tinkered around for a bit, but couldn't find anything other than a .xml file that didn't change anything. I know changing the sounds is possible, because I've seen it done before. I'm wondering if those sounds are unchangeable, or I'm doing something wrong. Help appreciated, thank you!
So I've been wondering about this for a bit. I got custom music working, so I decided I wanted to change the race finish theme, however I realized that was a sound effect. I then realized I have literally no clue how to do that. I tinkered around for a bit, but couldn't find anything other than a .xml file that didn't change anything. I know changing the sounds is possible, because I've seen it done before. I'm wondering if those sounds are unchangeable, or I'm doing something wrong. Help appreciated, thank you!