Sound causing volume to randomly reverberate.

[jenkins7]

So, I have a problem. When my friend types it causes my sound to reverberate and fluctuate rapidly. When any other friend types, it does not do this. I turned off volume attenuation, as well as my Windows call volume reduction. So, nothing else turns down my applications or sounds, other than this one person typing. Once I mute him, it fixes itself and I no longer hear it, so it is in fact him. He does not have priority speaker, or anything that causes it to happen. Out of my friends that all use Mumble, only about 3 of us hear it out of about 10. We try to get him to use push to talk, but he is stubborn. None of us use it, so it would not be fair to force him either. Is there any way to fix this? We've tried everything and I can not find any answers anywhere.