Long MP3 being muted half way through
I have a 37 minute video which I have added to a project, separated and deleted the audio. Now I am trying to add a 36 minute MP3 file to the project to be the audio, and the whole file appears in the timeline, but roughly the second half of it is muted. It's not being cut off, just goes silent.
I have tried deleting and re-adding. Any ideas?
iMovie '11, OS X Yosemite (10.10.4)