Iv had this issue several times and I want to get to the bottom of it. This issue specifically seems to happen anytime I get project stems from FL Studio. When I import the audio stems into Sonar Platinum (also used to happen on X3) they slow down. Iv gone back between both programs ensuring the sample rates and bits and all that jazz are the same. My work around has been to use an external File converter to convert the file into its same format (wave, MP3 etc..) then they import fine.

Also the timing is perfectly fine when just playing back in windows before importing into Sonar so it's not post export. It's when I import it. Also only does this in Sonar, they import into Pro tools and Wavelab just fine which pains me because Sonar is by far my favorite DAW.

What form are the stems in? Are they set to follow project tempo in any way?

Could something be changing your settings without your noticing?

What interface are you using?

Could be an issue with the output format. Couple of possibilities could be mismatched sample and bit rates between projects or output format has tempo held in file metadata. Are the exports in Broadcast wav format? Although Broadcast wav does not have official chunks for tempo (I think it holds timing and loudness metadata) manufacturers can add non official metadata in the file.
I am not an expert on this but have seen this happen before with other systems not just Sonar.

this happens almost every time i use FL Studio, which is why i rarely, if at all, use FL Studio

Any chance the FL export is the equivalent of a groove clip (or similar) vs. a straight Wave file?
 

Deactivate "Export Acidized" in FL - that's the problem. Sonar picks it up and turns it into Groove Clips and then it's downhill from there.

If it slows down without changing pitch, then it seems likely that "Groove Clips", as LJB says, is telling Sonar to change the speed.