Myself and one other person on the Steam LH forum seem to have the same audio bug, with an M-Audio Fast Track Pro USB 2.0 audio interface -- see the link. 

The sound periodically drops out, and my USB keyboard lights flicker -- it seems that the game periodically resets the entire USB bus, or something to that effect.

I know this is the classic "some old random piece of flaky PC hardware" situation, but nonetheless, this is making the game unplayable for me.  As an original pre-orderer of Elemental, I've been looking forward to playing FE:LH for literally years.  If anything can be done, it would be super awesome.

(In the meantime I will be trying to revive my PC's onboard audio....)