Note that there is cockpit speaker muting on the 737 when the handmic or boom mic is used. If headsets are worn at all times, however, then this would not be a problem (unless you had only one ear of the headset covering your ear and it happened to be faulty). Not sure how much the speaker mutes on a 737, but on some aircraft it can be considerable (basically no output).
Rgds.
NSEU