Hi There!
We might have experienced the same problem at a customer last week.
It occurs at a few 6921 phones randomly (4-5 out of 100), and the symptom is slightly the same.. no audio can be heard.
Network traffic capture from the phone's PC port (span to PC port) shows unusual high traffic, with lots of dupACKs and out-of-order packets.
I'll try firmware upgrade today, and return with any new thoughts.
regards,
Mark