Nothing long, maybe 2 minutes or so, just sound effects, no music. Growling, teeth gnashing, flesh ripping, snarling, groaning, screaming, that sort of thing. Some of my zombies will be carrying portable mp3's with speakers, and this will give them added flair.
If that doesn't work (or they forget batteries), I'll flood my whole Zombie Run with the sound. In that case a longer track would do better.