A lot of people import their audio files to iTunes in order to synchronize them to their iPod or iPhone. The conversion process takes some time, and it is slightly faster on Phenom than on the Athlon 64 X2. iTunes scales well with two processing cores, but typically doesn't benefit much from more than that.
Lame is one of the most popular MP3 encoding applications, as it can be highly customized via command line. It runs a bit faster on Phenom.