Tempo changes slightly when exporting with Sonar 8? Please help.

Ok so let me briefly explain my situation. I'm a rapper who makes my own beats and then record the vocals in a studio. We record using Sonar 8. When the studio guy sends me my vocals (WAV files), and I add them in my instrumental projects in FL Studio, the timing is off. It's only slightly off, but as the song goes on it becomes more and more noticable. Everything is onbeat in the studio, but here at home the vocals are slightly off. In the Sonar project in the studio, the BPM is the same as my FL Studio projects, so I don't know what's going on. Even more strange, (I have Sonar as well) when I put my instrumental and the vocals he sent me in Sonar, the timing is still off, but it's different. I was too fast in FL Studio, and too slow in Sonar. What The hell is going on? Anyone has an idea? This is getting really frustrating. Thanks!