I definitely know the issue is due to “slicing” on parameters between source instance lifetimes (sources are pooled). Conceptually, a given source has a bit of the previous lifetimes value and it “lerps” to the new value rather than instantly setting the parameter to it’s initial value. This causes pops for a bit after muting the audio as sources before the mute were non-zero and are lerping to muting.
It’s hard for me to know if you’ve got the fix properly integrated but that’s definitely the bug and it’s definitely fixed for 4.18.
I’d run the audio mixer with optimization disabled (Set OptimizeCode = CodeOptimization.Never; in AudioMixer.build.cs), then set some breakpoints (or add debug logging) to the code which is setting initial volume parameters, see what’s going on. Make sure that the parameter lerping object gets its initial value set immediately to the value.