Volume of audio clip visibly DECREASING without command

Hello All -

I have a strange issue. I am editing two hours of footage on iMovie shot with a digital camcorder and imported in via USB. I have a background track for the audio clip which is strangely decreasing in volume automatically, without command, at a certain point. As soon as it gets to a certain point in the footage, the audio file visibly drops to a lower volume.

I have checked the audio in iTunes and in its WAV format independently and there is nothing wrong with it. I have tried removing the first part of the audio and playing the part where it goes lower from the start and it plays regularly - so the problem exists at this particular point only (2.54). I'm not sure if it's a CPU problem to do with editing a lot - but it's really annoying. I have no idea. Even if I chop the audio it happens, and even if i drag the audio up to right underneath the footage as a clip it happens.

If anyone has any ideas on what's going on I would love to know.

Many thanks