Intermittent muffled audio

I've had my Macbook Pro, Version 10.13.6, operating macOS High Sierra for a couple of years. Beginning yesterday, I'm experiencing intermittent muffled audio when watching Youtube videos. It's random. I'm not adjusting anything anything when it happens. Just now when it happened again I tested it by listening to some music on iTunes, and the muffling was still apparent.

The speakers aren't blown, because when it's ok the sound is crystal clear.

I've updated everything, turned it off and on, but the problem persists.

Could it be a virus?

What's wrong, and what can I do to fix it without spending vast amounts of money getting it serviced?

Thanks for your help!