Comparing modified elements in Sets

[ChrisEdgemon at gmail.com]

I've got a set with contents like: [filename1.mp3, filename2.mp3,
filename3.mp3, ...]
I've got another set with contents like [filename1.ogg,
filename3.ogg, ...]
And another set with contents like [filename1, filename 2, ...]

I'd like to be able to compare set 1 with set 2 and have it match
filename1 and filename3, or compare set 1 with 3 and get back
filename1, filename2.  etc.

Is there a way for me to do this inside the compare function, rather
than having to make duplicate copies of each set?