Okeh...
I THINK I see this...except for one detail! Are you playing the tapes as though they were two-track stereo (each half of the tape is a different channel, with one being reversed...)?
If so, do you digitize the "backwards" signal and then digitally reverse it?
So you are looking for a sound-file application that can reverse a "backwards" (or, for that matter, any) sound file?
This leads to a couple of related thoughts...
1) Perhaps such an application could be packaged as "Is Paul REALLY Dead.exe"...?! There might be a demand among Beatles aficianados...
2) At the same time, I can't help but wonder what the results would be if other digital files could be reversed and run (other than about about a gazillion "Illegal Operation" crashes...)