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;