Sometimes I have noticed that the left and right channels of a stereo track are separately editable. For example, when dragging the edge of a clip to crop it only changes one channel. Also, the title of the clip show up at the top of both channels.
I can reproduce this by splitting a stereo track to mono and then making it stereo again. It also seems to happen sometimes when I paste audio into a track. Once a track is separated like this, the only way to fix it that I have found is to copy each individual clip to a new track and delete the old one.
From Audacity version 3.1.0 onwards, joining mono tracks creates a weird kind of “dual mono” track rather than a true stereo track. You can see the difference because the “joined” track has two “clip handles” (the “drag bar” above the audio clip), one for each channel, whereas a true stereo track has just one at the top.
To convert a “dual mono” (joined) track into a true stereo track, you need to use “Tracks menu > Mix > Mix and Render”.
Warning: When you apply Mix and Render, if your track has real-time effects, track pan / gain adjustments, separate clips, or envelope points, these will be “burned in” (applied destructively) to the new track.
There’s some discussion about how to sort out this mess: https://github.com/audacity/audacity/issues/2722 but as far as I’m aware there has been no work towards fixing it since that discussion started a year ago.
