Audio Settings
To look at how to play sounds in MonoGame goto the Audio Playback page.
Sound Effects
To set the master volume of all sound effects you can do:
SoundEffect.MasterVolume-=0.05f; SoundEffect.MasterVolume+=0.05f;
If you have set the volume of each SoundEffect this will be multiplied by the master volume.
Background Music
You will need to MediaPlayer to play and background music or song. You can set the volume of this:
MediaPlayer.Volume+=0.05f; MediaPlayer.Volume+=0.05f;