Michael,
One of two possibilities.
The first is the UV-5R (and variants) with the very earliest versions of N5R-20 firmware have a BEEP bug. If these radios have menu 8 (BEEP) set to OFF, then receive will be muted unless the [MONI] button is pressed.
The second possibility is that a CHIRP Radio Image (*.img) file from an older radio with BFB or BFS firmware was uploaded into a new radios with N5R firmware. If this (or the opposite, N5R into a BFB/BFS radio) is done, then receive will be muted unless the [MONI] button is pressed.
The latest CHIRP daily build will check to see if an incompatible image is being uploaded into a UV-5R (or variant) radio and it will abort the upload if it is. Unfortunately, the CHIRP stable builds do not have this same protection. It is for this reason, that I don't recommended using the a CHIRP stable build with any Baofeng/Pofung radios.
The workaround for the first issue is to set menu 8 (BEEP) to ON (enable Settings->Basic Settings->Beep using CHIRP) and leave it that way.
If the muted receive is cause by the user uploading of an incompatible CHIRP Radio Image into the radio, then the radio can easily be restored by uploading the original CHIRP Radio Image that was created/saved before the incompatible image was uploaded into the radio. If that "original" file was never made or is no longer available, you can visit the "Miklor.com N5R-20 and N5R-30 Issues and the Simple Cures":http://www.miklor.com/uv5r/UV5R-Recovery.php web page to get instructions and access to a collection of "factory" CHIRP Radio Image files.
Jim KC9HI