wav files in a session get corrupted - full level and distorted

Audition CS6; adding wav files [48KHz, 24bit] into a multitrack session [32 bit mixing].

Clips [files], but not all,  that have played properly in this session, now play very distorted, and at full [or very nearly] level.

We have gone back to the previously exported * session to recover and re-edit, but we really don't want this happening again.

( * I tend to do File, Export,  Session...  (having previously checked the box in Preferences to 'Associate all fiels with the session' [whatever] ) so that Audition creates a folder with all the needed files for that session next to where i save the session .sesx file.  )

Any and all ideas please!

Thanks

John