So, we have Zombies that are drawn instantly to sound.  How about we set some speakers on cliffs and let them do a lemming dance? Or dig some decent pits, put a speaker in the bottom and when the pit has enough of them, just bulldoze several tons of dirt on top of them? If you feel like that is too much, Why don’t just attract them to an open space and have a demolition rally using heavy construction equipment versus their butts? (I got dibs on the big roller)

So, an IPod with a speaker, heavy machinery and some diesel fuel pretty much solves the issue of thinning the heard of mass zombies.  You then can save your ammo for the occasional straggler, sort of varmint hunting tall pests.