Unfortunately I’ve found no solution or cause yet. I’ve since put the SSD (El Cap) in my Macbook and moved my HDD (Lion) to the optical bay (replacing the CD drive) so I’m running with two boot drives. I continue to use Lion to work in Audition (and Premiere) for now and until/if I find a solution. Would still love to use Audition in El Capitan though if anyone has some insight. For the record, I did upgrade to 16GB of RAM which didn’t solve that issue, not that I expected it to anyway.