There seem to be a few variables with the headsets in general, one being that not everyone knows how to set things up properly. I haven't been able to find the MGS one yet, but I tried a bit with my girlfriends, and nooticed some people were definately more understandable than others. I can only assume those coming across as mumbled static are setting their in game bletooth settings for broadcast way too high.
There are 3 settings.
-Sending
-Recieving from players
-Recieving from in game preset voices.
I think setting all to around 6 or 7 should be ok, but 9 & 10 put things in the static range. It'd also be nice if there was an in game audio test, like there is in the ps3's audio settings, so you can test how clear you're coming across.
I'm guessing over-tuning the broadcast sensativity also has something to do with some people's headset's randomly popping on & off, since that's what happened to me. All settings for the audio can be changed during matches, and inbetween.