How to filter out overlapping audio? Given a mixed track and one stem track, what's the best way to "subtract" the stem from the mix?
Inverting the background should work, that's exactly how a balanced audio connection works to eliminate noise, but how exact a match is your isolated background track compared to the mixed track? It's only going to fully work if it is recorded at the exact same quality, volume, etc, and the timing is precisely the same. What was the recording source for each track? How confident are you that the two sets of background audio that were recorded are identical to each other?