A friend of mine and I are recording an album together.  He's in Florida, I'm in RI.  He's using Sonar X2, I'm using Sonar Platinum.  I recorded mostly all the instruments of the song.... basically a backing track.  I did record to a click, which means each song has a specific tempo.  I sent him mp3's of the songs, he imported them and sang the lead vocals to them.  Then, he exported them as WAV files and sent that back to me, which I imported back into my project.  Should line up perfectly fine right?  Well, it didn't, unfortunately.  There was a noticeable difference in where his vocals were in relation to the music.  
 
Any ideas on what the problem is?  Does he have to set the tempo in his project to the tempo of my project?   

Is this situation simply off by a set of samples (I.e., you can nudge his vocal left/right to achieve a match)? The mp3 you sent will be off by some samples on the lead by definition, and what he sent back was tracked against it. If you can nudge and match (he sent you back a single broadcast wav file?), that is expected. If it drifts in and out, that is something that would need more detail of what is happening.

If he was working with audio only (no MIDI), the tempo of his project is immaterial.

What he said
 
FWIW - I like to use a sync tone on beat one two measures before the start of a tune.  All collaborators use this tone at the beginning of their tracks, providing a full length track from this point regardless of where their part(s) come in.
 
Files are obviously larger, but lining things up is never a problem.  Different folks use different DAWs, so we just exchange mp3 or WAV files via box, where we have shared folders.
 

There some things to consider.
- Sample rates  
- A possible fluxing CPU (that is on its way to the graveyard).
- Latency
- Can he sing/How well do you know your buddy
- Does your project start at 1:01 ? or 2:01 ? (this one is questionable) However I found when recording if there   is a slight pause when hitting start on the daw it can throw things out of whack a little bit.
 
As Bob mentioned I also use synch tones, click tracks when collaborating with others.
* - Latency
What are you / and what is your buddy using for soundcards ? Because if you are using the C600 and he is using onboard audio and there was latency, that can mess things up.
 
 

All of the above and. 
 
- Even though it's not technically necessary for audio, I would have him set his project to the same tempo. For midi it's absolutely necessary.
 
- Make sure when you export that your tracks start at the beginning of the project. Exporting a track that starts at 1:02 will be one beat off if you import at the beginning of a project. You'll need to grab the left edge of the audio in the track and drag it back to the beginning to pad it with silence prior to exporting it.
 
- As also mentioned, midi is tricky as the export will start at the beginning of the midi notes. Place a "silent note" at 1:01 to make an exported midi track start at the beginning of your project.
 
gabo

I find that when you import an mp3 you always have to adjust it a little to make it line up with the grid. Doesn't seem to be a problem with wave files so I use those for exchanging whenever possible.