This video shows how you can use the built in audio filters in iMovie 10.

Older tutorial about how to do this in iMovie 11: https://www.youtube.com/watch?v=f4CVklnSAJ4

This tutorial works from iMovie 10.0 to 10.0.x

Partial Transcript: I can go now to the filters tab right here.. By the way some earlier iMovie versions may have an adjust button over here and there you would get to this setting. Since this is a piece of audio, we’ll just click on the audio effects box and here there are many different effects you can choose from. All play them all…Overall its a pretty good feature to have especially since its built in.
Reverb audio is also included, Small Room, Medium Room, Large Room and Cathedral presets.