The volume bug that was recently uncovered for Galaxy Nexus has been acknowledged by Google. The random mute / low volume was a result of users using their phones on 900MHz frequency and it will be soon fixed as Google has confirmed to release the patch.
The Android PR team confirmed to Android Police that “they’re aware of the volume issue and have developed a fix.”
Although there is no word on the exact release I presume it should be out pretty soon. Till then refrain from using the phone on the 900MHz frequency.