Well, that is what it was folks. Certain songs were somehow saved with the volumes set to zero and pans set to -63.

I reset my volume and pan settings for each instrument and right clicked on COMBO to save settings to default mix.

I took Rachael's advice and utilized the file utilities command to change all the songs in the current folder to default mix...
While it was doing the conversion I could see the volume settings for each song flash before me very, very quickly as the default settings were applied to each song in the folder and A LOT of the songs in the folder had the volumes set to zero. Not sure how they got that way (that's another thread), but more importantly the problem is resolved.

As we used to say "you've got a short between your headset!" Yea...me!!

Thank you for your responses.

Mark