Open the song so there are two blue waves in Audacity.
Use the drop-down menu under the left black arrow > Split Stereo Track.
Select the bottom track by clicking just above MUTE. > Effect (top menu bar) > Invert.
From the top wave drop-down > Make Stereo Track.
That should have reversed the two. Nobody wrote that the karaoke is going to suddenly start working. This is just solving the original problem. Most stereo songs and all mono songs can’t be turned into karaoke tracks.
Koz