It's the same whether storage is internal or on an SD card. As I've already explained it seems to happen around the 50 second mark. The sound anytime before that is perfectly clear. Then after that time frame it gets muffled and sounds like it's underwater. I've never experienced this with any phone that I've had before so I'm kind of stumped.
I need to add that this is a brand new phone I've only had it for 5 months and ever since I've had it and tried to video record something it's been doing this.
It's probably a firmware issue. I'm not sure how easy it is to reinstall firmware on LG phones. Is the phone still under warranty? If so, your best bet would be to contact LG for warranty repair/replacement.