The whole point of megamixes is that you can’t tell exactly where one song ends and the next one starts. It works - Audacity has no idea where one song ends and the next one starts.
Command + “.” (dot) to add a label during playback.
Command + B to add a label at the cursor position / selection when stopped.
Click in the small circle in the label to drag it to a new position.
then knows when songs change from one to the other
Audacity can’t do content.
Audacity only “knows” the up and down blue waves. It doesn’t know what a song is or who sung it. Audacity just knows that the blue waves last for three minutes and fourteen seconds.
It’s the same problem with Noise Removal. People are surprised that they can tell when the airplane flew right through their interview, but Audacity hasn’t a clue.