How can I create a custom alert sound in Mountain Lion? I want to use a personal sound file (currently an mp3) instead of the system alert sounds (sosumi, basso, etc.)

I remember this being really easy on Macs a couple of decades ago, but can't figure out how to do so today.
You need add the sound file to /System/Library/Sounds/, or ~/Library/Sounds. (If you don't know how to locate that easily, you can search for "sosumi", and follow the breadcrumbs at the bottom of the finder window.)

The sound file needs to be converted to AIFF or WAV format. Macworld has a good writeup on how to do this using iTunes or garageband.

Then, in System Preferences-&gt;Sound-&gt;Sound Effects, it will show up as an option for your system alerts.