Audio file not found code 21

The location of your audio should be contained within the session file - assuming that you recorded in Multitrack view, which it sounds as though you did. If you recorded to a remote drive, then this needs to be connected, obviously. But if you didn't elect to keep a copy of the recordings in the folder you set the session up in, then yes you may have to relocate them. We can't tell you any more than that because we have no idea of how you have your machine structured - but Audition doesn't lose files, as a rule - they will almost certainly be where they were recorded to, even though you aren't currently aware of the location...