Make sure the song you're playing does not have a low volume e.g. calm music
Make sure both master volume and music volume is not low
There are three things you can do
Go to settings (Ctrl+O) and type "volume" to find these
Use your mouse scroll wheel to make the volume go up.
If you disabled mouse scroll wheel while playing, make sure you're in the main menu screen and not in the song list and/or playing a map
Use the arrow buttons to increase the volume while in game. Arrow up to increase, Arrow down to decrease, and left arrow to choose between Master, Effects and Music volume.
