Would it be possible to lay the source music over the Thief audio, lining the music up so that lines up with the background music running beneath the spoken words? Basically, so that the good quality music for the most part blots out the lesser quality background track, and the only audio we're hearing from the video soundtrack is what is spoken by the characters?

Of course, if you're re-editing scenes where the music is used, that might become rather nightmarish...hmm. Do you happen to know any audio experts that would be interested in working on this project? You could focus on restoring the video, while somebody else could put together the best possible audio track?