Convert it to the Xvid format. The best way to do this is:
1. Download and install the Xvid Codec
2. Download and install VirtualDub
3. Open the Video in VirtualDub
4. Video -> Compression
5. Choose XVid and press OK.
6. Audio -> Full Processing Mode
7. Audio -> Compression
8. MPEG Layer-3 56 KBit/s
9. Save as AVI
And there you have it, an Xvid codec'd video with MP3 Audio.