Sprint's Samsung Galaxy Note 3 gets maintenance update to repair audio bug
Some owners of the Samsung Galaxy Note 3 have been complaining about hissing, popping and garbled speech during phone calls and this update should take care of it. While the update is being sent out OTA, Sprint's website cautions you that updates sent in stages could take several days to arrive on your phone. If you haven't yet received the notification that it has arrived on your Galaxy Note 3, you can try to coax it out by going to Settings > General > System update > Update.
source: Sprint via AndroidCentral