I'm really not that familiar with that kind of applications, but I have noticed that PCDJ tends to at least advertise their product helluva lot and it seems to be able to do the mixing as well. http://www.pcdj.com/
Decode the Mp3 file to .Wav file(s) by AudioCatalyst and then mix .Wav file(s) by a Application provided by some sound card manufacturers or something like that, and encode the results back to Mp3 by AudioCatalyst again.[the most foolish way to finish the task.]
bpm studios works with mp3's http://www.bpmstudio.com/index2.html dj software you can download a demo if this helps you please listen to one of my songs zoltanrocks in the rock section