I help run one, but it's not along the same lines as mp3.com plus it's still small (<100 members as of yet)
After submissions are made, members will listen to and review the submission. At least in theory. Sometimes they get lazy and don't do squat except post on the message board
In best circumstances though:
You upload an mp3 and we resample it, if needed, into a lofi sample rate. Alot of people are still on dialup and a 10mb mp3 is too much for 'em to take time with. So we resample it to get it under 1mb. It takes away some of the sonic clarity, I know, but people just need a good idea of the music itself.
The mp3 is embedded into a flash player to prevent unauthorized downloading for the protection of the submittor. As with any site though, if your material is not copyrighted, then we cannot claim responsibilty for it.
Work is currently underway to make all of the submission pages a protected area. Gotta love phpnuke
If it doesn't work - scream, curse, cry, and whine profusely. Just don't let the wife see it!