I just had a four day trip with a guy with the QC2/IFM and we had this problem. I don't know if it was because he wasn't kissing the mic or what, but it was an added stress several times. Almost screwed the pooch and taxied across an inactive runway we were told to hold short off. I alway try to listen in closely to the controller, listen to my FO's read-back, and then I repeat the instruction to him. I figure hearing it three times should set the stage for us to do WTF we're supposed to do. I read back to him to taxi to our assigned runway and he didn't catch that I didn't read back a hold short of the runway in-between. He had just exchanged his set-up and it was new out of the box with a fresh battery. His last Captain mentioned the same problem about not hearing what he was saying and he had sent it back hopeing to fix the problem. He also mentioned that his side tone was almost non-existant. Next time I run across one of these with this problem the guy is going to spend 3.9 days using a maintenance loaner headset.