Hmmmm...
Sounds like it's worthwhile to figure out what for sure happened.
So... when you audition (playback) the audio in Sonar it's normal.
When you audition the file you export it's fast.
I know that the RME can be forced to a different bit rate by an instance of the RME Digicheck software and cause goofy speed and pitch changes when auditioning audio. For example... if I'm using Sonar at 44.1 and I hope RME DigiCheck to monitor the FFT and the RME is set to 48, I'll suddenly be listening to a file that is at 44.1 at the rate appropriate for 48. This changes the pitch.
The fix for me would be to alter the sample rate within DigiCheck to match the Sample Rate of Sonar.
In you case, you have Sonar working at 48. You export to 44.1. Is it possible that when you play back the file exported at 44.1 that your device is still stuck at 48?
If you burn it to CD does it sound normal?