When you say tracks do you mean songs or individual channels of a multi-track recording (one track is guitar, one track is bass, one track is lead vocals, etc...)? If it multi-track, I would say convert the MP3 to WAV and mix away, but notate the source information in a file. If it is individual songs and you are sharing online I'd say leave the MP3 as MP3 and the WAV as WAV. If you are burning a CD, convert the MP3 to WAV to make the CD but again make a clear note in a cover insert or label included with the CD which tracks are from which source.