How to Add Custom Ringtones to a Wireless Doorbell by DIYHacksAndHowTos
What You'll Need:
Wireless Doorbell (and required batteries) 9V recording module (RadioShack Model: 276-1323) 9 volt battery Wire strippers Wire Cutters Soldering Iron and Solder (optional) Hot glue and hot glue gun (optional)
Seems reasonable enough for anyone comfortable soldering and doing a little rewiring. Of course, you can skip jumping through all the DIY hoops and buy an off-the-shelf MP3 ringtone solution with similar features (which I'd probably do).