TCL TV using Roku has a know problem with low audio on Netflix when using HDMI-4 and their E-ARC. Audio is fine if not using an external amp, however if you intend on watching a lot of Netflix through a home theater system, you may want to look at another brand. Prime however works just fine. I believe it to be a software problem with TCL.
I have a series 6, SW version 9.4. bought fall 2020