Drag and drop your music onto your timeline,
and you'll see it appear below your images and videos.
You can use in and out points
by pressing I and O on the keyboard
if you want to add a specific section of the track.
Split the audio clip the same way as video
by moving the position marker to the point you want
and then pressing command + B on your keyboard.
Change the length of the track
by dragging the arrow icons, like this.
To change the volume of the music,
click and drag the horizontal line on your track up or down.
You'll see the percentage change,
reflecting the volume.
To gradually fade in or fade out of a track,
click and drag the two circle icons
on the ends of the music, like this.