Use WAV or FLAC. MP3 compression often "smears" the stereo field, making it harder for Audacity to distinguish between center and side audio.
The Audacity Vocal Reduction and Isolation effect is a powerful built-in tool used to separate vocals from background music. It works by exploiting the way stereo tracks are mixed, specifically targeting sounds panned to the center of the stereo field. vocal reduction and isolation audacity
Effect > Vocal Reduction and Isolation.