Vizio released an automatic update that broke the HDMI 5 port with game mode on. They have not released a fix in weeks, and have not given a date for a fix.
sources: https://www.reddit.com/r/VIZIO_Official/comments/bep2r3/no_audio_when_game_low_latency_is_on/ https://www.reddit.com/r/VIZIO_Official/comments/bly4qw/can_we_get_an_update_on_the_audio_lag_from_last/