Recently Maxis has posted a work-around for the infamous karaoke bug in Nightlife. The problem with the karaoke machine is that some people don't hear the voices of the Sims using the object. MaxoidKane has now suggested on the official BBS to change some of the audio settings. It might work for some people, but maybe not for everybody. The problem is that the game doesn't play some sounds to improve performance, but as the karaoke sounds (and sounds from the band objects in University too) are quite long it is easily noticeable if these are missing, whereas usually only short sounds are not played. Setting the Audio Quality to the highest setting, in the Options panel, will make the game play most sounds, but it may also affect performance. See MaxoidKane's post for more info.