I have read old posts on this issue and really cannot find any solutions that work.
I have the X1 platform. Normal volume for watching FIOS programming is acceptable. Way lower than other providers I have used in the past.
The volume level using the FIOS YouTube and Netflix widgets is painfully low and muffled. I run from the X1 box through a 5.2 amplifier to my HDTV. All via HDMI.
When I stream YouTube and Netflix from my BD player and Roku the volume is stellar. I set my volume ( range is 0-60 on amp) to about 20-25 and it's loud and clear. When I use the widgets via FIOS I have to set my volume to 50-55 and it's still very low and poor quality. Unusable basically.
I have tried all audio setting: both variable with heavy and fixed set to 30 with heavy. No change at all. So right now I have to steam Netflix and YouTube through other devices.
So basically the Netflix and YouTube widgets on FIOS are useless due to such low volume levels. And seeing that I have no issues when using ROKU and a smart BD player I know my setup is fine.