I had the exact same desire; I don't care much for voices; just wanted different tones for each. I didn't see a good way to do this directly with the panel, so I picked up an elk-124 voice recordable module and elk-129 module for programming the sounds off a computer or other sound source. I wired it inline with the elk's output to the speakers. I hooked it to the 7 or 8 onboard voltage outputs on the Elk to make things pretty easy.
I haven't actually gotten around to recording the tones specifically for each door; but I did use it to record my doorbell (had fun with jetson's doorbell for a while before settling on something more traditional). Also use it around Halloween - custom doorbells on halloween, and alternating creepy sounds that fire off when the front door is opened - pretty handy all in all.
It's a little pricey for what it is, but I like being able to record absolutely anything I want to be triggered on demand. With the added bonus of having my doorbell in the system, I can use relays/resistors to create a "quiet mode" that cuts off the upstairs speakers when babies and wives are sleeping.
