[Tom McCreadie]

Does Audition have a convenient way to split a stereo track into two
mono tracks? That might give you some more insight.

Yes it does..and I'd urge Gary just to split that problematic stereo track
into two mono's..then throw one away! :-)
He only needs one mono vocal track for the Audition mixing operation with the
band's stereo track...and he can suss out the misconfiguration issues later at
his leisure.
--
Tom McCreadie

Live at The London Palindrome - ABBA