FCPX v11 - Sporadic Music levels

I have not experienced this problem.

Try deleting preferences (always a good thing to try when the application misbehaves).

To delete preferences, hold down command and option while starting the application and click Delete Custom Settings. The application will start with default settings and open an "Untitled" library. Adjust the settings, reopen the library you were working, and see how it works now.

As a temporary troubleshooting step, disable all external audio, both wired and wireless. No headphones, external speakers, AirPods, etc. Use only the basic built-in speaker on your Mac or monitor.

Then select one clip and use the Audio Inspector (CMD+4 toggles Inspector, then select audio tab) to examine any effects on that clip. If it's multi-channel audio, you may have to select the specific audio channel waveform in the audio inspector, since audio effects area channel (aka track) specific.

Make sure there's no noise reduction effect or similar audio effect applied. If the voice isolation effect, it can cause music to vary in volume.

Even if you never intentionally applied an audio effect, just clicking on the "magic wand" in the audio analysis section of the audio inspector may automatically apply items like loudness or noise reduction.