If you are wanting me to play on a recording (i.e. your song), and not just some lines or unaccompanied piece, I will need an MP3 file. Make sure the file you send is what my part with be mixed will on your end. Send me the entire MP3 file, so when I send you my recording, the timing will match up perfectly. If you have any other music, charts, notes, etc. that are needed for the recording, send that, too. Clear, concise communication will help get you want you want quicker.