Trimming audio clips in iMovies

New to iMovies and having trouble with adding audio. Can't find anything under search or maybe I'm not searching for the correct terms.

I have 3 songs I want to use in the background but iMovies won't let me add them where I want to. I'll add a song from My Media and the audio will appear under the pictures. iMovies doesn't let me drag the beginning or end of the clips where I want them. It's like there's a hard wall at certain points in the video. If I drag a song beyond it, iMovies will "flip" the whole song to the other side of the this "wall".  

What am I missing?