Ive had this same issue happen to me, I noticed this recently over the last week.
Its happened to me on youtube everytime it autoplays the next episode. Its happened on netlix when first loggin in and playing a movie, havent tried a series to see if it does happen when next episode plays.
I have to open the options, go to volume mode, switch from leveling to off and back to leveling to get the audio to play.
I have not had a problem when I switch to an HDMI output with a game console.