Audition should not be crashing as you describe. If you can send a crash log file, we might be able to see what is invoking this behavior. You can send the log file with the following steps:
1. After Audition crashes, launch Applications/Utilities/Console.app
2. Open the "User Diagnostics Reports" group and locate the most recent entry for Audition
3. Drag this to an e-mail and send to firstname.lastname@example.org
If you can also include your session .SESX file (not the media) this can be helpful in troubleshooting as well. Are you using any third-party plug-ins, or just the standard Audition effects?