I disagree about the singular use of MP3 killing it. Personally if I were discussing anything regarding MP3 it would be the format itself, the legal situation, copyright issues, blah blah blah rather than discussing individual MP3s, thus making the singular a better name IMO.
In fact, I think DiscussMP3s would be an incorrect name for a site discussing songs as you're actually discussing tracks/composings/productions/music, which are certainly not limited to the MP3 format.
That said I don't think the domain would have too much value undeveloped. If you can put forums up and generate a bit of a community it could indeed yield a good value. As is -> Low $xx
MP3 works when its with the right name, i dont see it fitting with this name, sorry bad reg, sell it on the NP auctions or develop it and it will fit, anything developed with traffic will fit