I've been combing these forums and tried the reregistering of codecs, reinstall of Media Manager and looking at the log files (C:\Documents and Settings\"Name"\Application Data\Verizon\Verizon Media Manager\AudioTranscoder.log), yet I could not play back mp3s, wav files, or even the sample audio files that come with Media Player (v9.2.70). The log files looked clean. The STB looked like it was streaming the audio just fine, yet no sound. My PC is Windows XP SP3 and my TV is hooked up through a Bose system using only HDMI cables. Here lies the issue.
Media Manager can play my home videos, home pictures, but no music files using this configuration (HDMI). I found a workaround. I kept my HDMI connection between my FiOS STB and the bose system, but I added RCA connections from the FiOS box (STB) and the Bose using the Bose AUX input. Switching to the AUX input, I can now play the music. This is a kludge to use a technical term and I'm going to call FiOS and see if they have a more real solution for this issue.
I have unplugged and replugged the power to the FiOS box (STB). Same results.
I'll post here with any updates I get.