So I don't know if this questions was answered or not in this method because I don't have the time to read it right now, but with Audacity, you can find an area of the music with no singing or music (what would be silent if there was no hum) and then highlight that with the cursor. Then hit "effects" and select "noise removal." First, hit the box that says "get noise profile," and then highlight the entire recording and go back to effects and noise reduction and hit "ok" this time. That would get rid of the effect on a recording at least. It also works with record clicks.