I use Foobar as my primary music player, and I have for years. Recently I noticed something odd: the volume was constantly changing, as if someone was subtly messing with the volume slider. A bit of googling revealed the solution, if not the reason:
I set the eq to boost the bass. Something in the system got overloaded when there is a high bass (drums for example) content and self adjusts the volume. This means that the volume on the mid-band drops when for example the drums or any low frequency content increases. There is some lag in the auto volume adjusting system hence the volume yo-yo up and down as the music plays. Source
It doesn’t make clear what component is doing the compensating, whether it’s part of Foobar’s equalizer DSP module, or something in Windows (Win7 in this case) or what. But the solution holds true: flat-line the EQ and it stops being a problem.