The sine-wave-track is a workaround to make Audacity’s Native AutoDuck into an envelope-follower.
e.g. AutoDuck the constant-amplitude sine-wave-track, then use that track as a control-track to Autoduck other tracks, which will then have the same envelope as the original audio.
The simpler way is to use an envelope-follower, e.g as provided by Steve’s Dynamic-Mirror plugin.