Really enjoy the game so far but the audio and music is too loud during the middle of the night and I want to have the option of lowering it within the game itself without needing to mute my computer system if it's daytime. If it already exist, I have no idea where to find it.  

As far as I know there's no option to lower the volume, you can mute it in that bar at the top or lower it in your PC using the volume mixer.