For the "final lap" music (used the most of time for Mario Kart Wii), you have two options: using another song or speeding up the same song.
- You must convert your BRSTM to WAV format by using brawlbox to open with Audiacity.
On Audiacity:
1. Open your song
2. Select the song
3. Click "Effect" on the toolbar at the top
4. Click "Change Speed"
If you don't want the pitch to change, and only want it to play faster, then click "Change Tempo"
Drag the "Percent Change" bar (~20% is a good recommended value for changing the tempo)
5. Click "OK"
Note: this is from wiki.tockdom.com