I am a newbe to processing timelapse, I have lightroom 4 and LRTimelapse 2. I have processed and exported my first video. My question is how do I add a sound clip to my finished video..

If it's only one clip you can as add the music via the Lightroom Slideshow module.
Normally however you will have several clips you want to combine, than you will need a video editor for this. There you can add music as well.