Been trying to figure this issue out for couple of weeks now and think I have it narrowed down to VoiceAttack.
I switched from my old analog 7.1 headset to an usb wireless one and after that I noticed some sort of random "normalization" going on when using VoiceAttack with HCS Voicepacks (Verity for Elite Dangerous) and EDDI plugin.
I'd set the volume levels to desired levels, but randomly VA responds to commands would be extremely loud, like the volume would be set to max. This would alternate between HCS voicepack (pre-recorded responses) and EDDI plugin (TTS engine doing the speaking).
I sort of fixed the issue by separating EDDI from VA and running it standalone along the side, so I have separate volume control for them. But still I occasionally get some response from VA coming extremely loud. And it's not a specific command, just randomly any response might suddenly have it's volume at maximum. It's like there would be an loudness/sound normalization going on, except it comes and goes.
I suspect it's the VA audio options that somehow mess with the Usb audio.
Only have this happen in VA, plus I've made sure no audio enhancements or effects are enabled, uninstalled and reinstalled audio drivers, etc.