We've left adding the soundtrack to last, although this brings us back to the first Import stage of WMM's three-part process. Use the Audio or Music entry under Import to bring in your desired sound. Make sure you are in Timeline mode, and then drag your audio file from the library onto the Audio/Music track. You can then drag it about to the position you want, and drag the ends to fit.
Right-click on the audio and choose Volume to adjust the loudness. Another option is to use the Audio Levels option. This can be found in the same drop-down list used to switch between Storyboard and Timeline mode. The simple slider balances relative volume between the audio from your video and that from any added music.
You can also add Fade In and Fade Out. However, this operates across the entire clip. To add a Fade In just to the beginning of your audio track, move the green time marker to the point where you want the fade to end, make sure the audio track is selected, and use the Split button underneath the video preview window to break the audio track up. You can then apply the Fade In just to the first section. Use a similar method to add a Fade Out at the end.
You can also add your own narrative to a WMM project. Switch to Timeline mode, then choose Narrate Timeline from the same drop-down list. Set up your microphone, press Start Narration, and record your vocal musings as you watch your video play back. The results will automatically be added to the timeline in the correct location. However, as there is only one additional audio track, you can have either a narrative or music soundtrack - not both at the same time.