The easiest way to edit the sound is to convert the 5.1 Dolby Digital track to 2.0 WAV, and splice in the alternative audio for the section with the song you want to change (assuming the timings are the same). Then encode the alternative track back to AC3 and mux with the converted video.
If you want to keep the 5.1 mix, you will need software that can edit multi-channel audio (e.g. SoundForge 9).