Interfered Backing Track being used if not enabled

[Lumaah]

If you first interfer the back tracks of a song, then do it again without enabling the interfer back track option, it will still merge the interfered back track and not the original one.

[ShiromiyaG]

Sorry for the delay in replying, I was taking some time off. Can you test the last commit? I think I've fixed it. Before testing, delete the backing vocals file if you're going to test it on a song you've already made

[Lumaah]

nope, they still got merged