First of all we need to know what Zuki really wants to do i.e. Mix or Polish.
As Diki said if you wanted to Mix the song then you would need to use the Pa800 'sequencer' to record the song, then Mix it within the Pa800 sequencer after recording it OR save the song file as an SMF and do the Mixing on your DAW software on the computer. Sonar LE is a good one for that. After you Mix it (whichever way you choose to Mix it i.e. computer or pa800) then record the finished Midi into a .wav or .mp3. You can then use a Multi-track recording software to add the vocals to the song. For example PowerTracks PRO Audio from PG Music is relatively inexpensive and works as advertised.
If you want to "polish" an already created and existing .mp3 then disregard the last.
There are many software programs to 'polish' your .mp3 or .wav. I use Sound Forge 7.0 but it's rather expensive. A free one as already mentioned is Audacity. Sonar LE is another one and is less expensive than Sound Forge. Plus the other ones mentioned within this thread.
Best,
Mike
[This message has been edited by keybplayer (edited 05-15-2008).]