I believe it is the music volume, I re-started the game in the browser, and opened the config menu to check and this is the position of the music volume slider:
and I wasn't sure if this was intentional since the sound volume is set at max. (during my playthrough I was wondering why I couldn't hear DreamByte's music for a second until I checked the settings, but I assumed it was to prevent it from playing loudly when the game started?)