Follow this procedure
Tune to a SD channel – Turn on the TV and turn down the volume.
Using the FVP remote turn up the box volume to near max. Now use the TV audio volume to set a level that gives you a reasonable normal volume level. As Trev says thereafter use the TV volume or get a logitech Harmony remote which lets you substitute the TV as the volume control device.
Now Press the FVP button and choose settings.General Settings – Audio. OK
If you use HDMI to send Audio to the TV – Change HDMI to Stereo
If you also use the optical TOSlink output to feed a sound bar change S/Pdif also to stereo.
Tune gto a HD channel and check the volume level is similar to the SD one you used originally.
Note If you use a power on timer make sure that you set the same box audio level in the timer settings. If you don’t the box will revert to the default 50% settings (A factory reset also resets the level).