Recently I had to perform something as simple as importing MP3 file onto an audio track in Cakewalk Sonar 8.02. Apparently, my MP3 file was not standard enough, making Sonar totally unhappy.

After doing a bit of searching on Cakewalk forums I stumbled upon this thread which, I hope, you will also enjoy:
Importing MP3s. After following an advice by one of the posters, I was successfully importing my MP3 files just a couple of minutes later using the famous K-Lite codec pack. Give it a shot and you won't be disappointed!