I'm having the same problem with the YouTube widget on the Fios box. Verizon support says they fixed it but didn't.
Check the audio settings and make sure you choose Pass-Through. That may help.
Go to the settings, then go to audio. From there, there should be a "Fios TV One Volume" option. Turn it all the way up. That solved my issue.
Unless you need to control volume with the Fios box, you should set that to fixed.