I don't know of any problems that are specific to the 7931.
If the problem happens from IP phone to IP phone then I wouldn't worry about anything on your gateway.
Some things I would try to narrow down the issue:
-Give users complaining of this problem on 7931s a different type of phone, see if it continues
-Switch a 7931 not having the problem with the one(s) having the problem
-If the user is on a headset, try using the handset
-Upgrade the 7931's to the newest firmware
It's either a bad phone, headset/handset issue, bug on the 7931, or a soft-speaking user :)