We taxied across it the other day with the marker volume turned half way up (A320) and heard a faint whispering. Quickly turned the volume up to full and just caught the end of the message. Something about ATC clearance.
In theory it seems like a great idea, in practice there will obvioulsy need to be some kind of calibration for different aircraft types, those who use ANR headsets on the ground etc.
Just my 2 cents worth!