Why Does My Audio Decrease Dramatically After Upload To Youtube?

Audio question.

When I listen to a movie I've made in FCPX, and then listen to it again after I've exported it and it's on my hard drive, the audio is of a good, easily-understood level.

When I upload it to youtube, however, the audio become super soft and hard to understand. It just seems like the volume slider goes down to practically nothing, even though it is not.

You can see what I mean in this video:

I record my audio with a lavalier mic, if it matters.

Any ideas of what may be going wrong?

Thanks.

When you open the large audio meters in FCPX, be sure your audio is averaging over -12db and under -3 db. What is it reading now for this shot?

Thanks for the suggestion. I increased the audio so it fell in that range. I'm going to try to reupload and see what happens.