Interesting, so to be sure I understand: you're experiencing volume fluctuations with some videos and there's nothing showing in the audio path?
Yes.
Did you have a look at analyzer to see if the signal was at or near clipping?
The signal doesn't seem to be near clipping. All the spikes are rather below the ceiling.
What kind of output settings/devices are you using? One other though: some audio interfaces have built in "clipping protection" which is sometimes overactive. For example, some Asus soundcards will automatically turn down the volume when it gets close to clipping unless you bypass the Asus mixer by using WASAPI or ASIO. Maybe investigate whether your device has something similar, or lower the volume slightly in the windows mixer as well if you're not using a direct output mode?
I have Creative xfi Surround 5.1 USB soundcard SB1090 [the earlier model, not the Pro one]. In the output settings, I have selected WASAPI exclusive event style with the default 100 ms buffer. I tried lowering the volume in the windows mixer, but it didn't help.
I think I should mention this issue as well that I get a static noise often when the playback starts and sometimes during seeking. During the playback, there is no such noise. However, I don't think that this problem has anything to do with the above problem. Also, when using MPC-HC with ReClock (WASAPI exclusive mode), I don't have any of these problems.