Audacity automatically mixes track volumes to give an overall constant volume, whereas sometimes I want it to fade out or in and to provide dynamic contrast. It didn’t do this before, so I suspect there is somewhere in the default settings that an option exists that I need to know about.
We may need to know more about what kind of mixing you’re doing…
You can use the [u]Envelope Tool[/u] to fade-up and fade-down. The “trick” is to leave the envelope end-points unchanged and fade (quickly or more slowly) to avoid sudden jumps in the sound.
Or there are fade-in and fade-out effects.
A multitrack [u]DAW[/u] has better tools for mixing and volume automation but they are about 10 times as complex as a little audio editor like Audacity (and the good ones are not free).
Note - Mixing is done by summation so you have to be careful about clipping (overload distortion). You can lower the volumes before mixing, or you can export to floating point WAV (which can go over 0dB without clipping), then re-import and use the Amplify or Normalize effects to bring the levels down.
I think you’ll find it’s Windows, (not Audacity), equalizing the loudness level …