I was having the same problem and finally came up with a workflow that seems to resolve the issue:
1. Connect your CF card to the computer. Do not attempt to copy the files to a hardrive, keep the original media.
2. Open Canon XF Utlity. If you dont have it, download it from Canon's website at the support section for the Canon C300
3. Backup all the files in the CF card (or select files individually) to a hardrive
4. Open Premiere CS6 and create a new project with the proper settings. Dont forget to select 24p as the frame rate.
5. Using the integrated media browser, which can be found in the window menu, select the backup folder.
6. Select File>Import from media browser
7. Open a clip from the project window, sound sync issues should be resolved