After doing a little bit of research, I've managed to get custom music into LEGO Racers 2.
The music is 22050Hz Microsoft ADPCM in a WAV (Microsoft) container.
If you want to create your own, here are the instructions. I will be using Audacity 1.3.13 for this.
Open up your desired music in Audacity and set the Project Rate to 22050.
Go to "Tracks > Resample" and choose "22050".
Go to "File > Export" and set the "Save as type" dropdown to "Other uncompressed files".
Click "Options".
Change the "Header" dropdown to "WAV (Microsoft)" and the "Encoding" dropdown to "Microsoft ADPCM".
OK the options then save your file with the name of the game's original music file.
A file in the "LEGO Racers 2game datamusic" folder, called "readme.txt", will tell you what music is used where.
If Audacity says "Are you sure you want to save the file under this name?", click "Yes".
Cut/Copy your music file and paste it into the "LEGO Racers 2game datamusic" folder, overwriting the old one. (You could backup the original music if wanted.)
Run the game to see if it worked. If it did, well done! If not, make sure you followed the steps correctly and try again.
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