Look at https://www.cnet.com/forums/lg/ and focus on the posts with AUDIO in the title.
It appears LG can't fix this one. I'd go for a return to try another model.
I bought an LG55C7P in November and have been experiencing intermittent audio dropouts. The are random and brief (I'd guess 0.5 to 1 second). They happen most often when watching cable TV (Comcast). This problem does not appear to happen when streaming movies from iTunes and never happens when streaming music from Pandora via Apple TV 4K, nor does it appear to happen when playing a DVD. It has happened when streaming from Amazon Prime.
I've been in contact with LG and they said that the most recent firmware update (40.70.36) should fix this problem. While it fixed another problem (occasional no sound from Dolby Vision & Dolby Surround sources), it did not cure the intermittent audio dropouts.
I've switched out cables and done other testing which determined the problem is with the TV and not with ancillary devices. My best guess is that this is a firmware issue, and I will follow up with LG - but I'm wondering how common a problem this is and whether anyone has any other ideas.