I agree, Ted. Send them an MP3 for them to import, and then they can open a new track and record their part. Once they've recorded their part, they should delete or mute the track that has your parts, then export their recording as a new WAV file, and send that new singled out track back to you. Then you import that new WAV file into your original file, and it will be a new track. Highlight it all, and move it to the right place (because it will probably be off...the vocals or whatever will start at the wrong spot...but when you move it to the right spot, everything else on the track should align just right).

http://www.soundclick.com/MarkKaufman