In the disco scene DJ Steve Gad was playing 'Jump around' by House Of Pain (which is why the actors are jumping) but the rights to using the songs was just too expensive, so another track was used in post production.
Steve Gad, who played the DJ also played two more parts, providing the voice of a drunk who calls out during the cross carrying scene "Where did you pinch that from?" and in the scene where they are singing in the street, a voice can be heard shouting "Sing something we know". Both lines culled from a lengthy post-production ad-lib session by Gad.