From everything I've read there doesn't seem to be a fix for this, but I thought I'd try.
We just bought a 55" TCL 55R635 2021 model TV with a TCL Alto 8i soundbar. Every time I watch any channel on the Roku TV itself, the video is silent for the first second or two before the sound kicks in. Once the sound starts it is synced, but it doesn't start when the video/movie starts. This problem is not present in any of our hooked up devices. For example if I watch YouTube on my PS5 it plays perfectly but on the Roku TV channel the video is silent for the first few seconds. This is true for all Roku channels I have (Netflix, Hulu, Amazon Prime, Dinsey+, Viki). They work fine if streamed through another source like the Playstation but have no sound at first if played through the Roku TV channel.
I have unplugged the soundbar to use the TV speakers and there was no delay. The problem is with the soundbar.
I have tried restarting the TV. Restarting the soundbar. Factory resetting both. Changing every sound setting I can. Nothing seems to fix this. Any help would be appreciated as the whole reason we bought a Roku TV was so we didn't have to have the Playstation on all the time to watch things.
So, in the end we weren't able to fix it. Switching to optical input instead of HDMI ARC helped a bit (changed the skipped audio from 2 solid seconds to microseconds, but it was still noticeable). After a lot of digging and cross referencing we basically came up with 2 theories:
1) The Dolby Atmos actually is not compatible with or supported by our TCL Roku TV. We think the delay might be the soundbar's brain trying to decode the sound accordingly to check for Dolby Atmos (even when audio settings were on stereo and passthrough)
2) Even though this TV says it has eARC, it really only supports regular HDMI ARC.
The Verdict: there is no fix for this. After a lot of research we settled on the Vizio V21-H8R soundbar to try instead because it has no Dolby Atmos, regular HDMI ARC instead of eARC, and has a setting that let us adjust Audio Delay into the negatives manually. The Vizio soundbar came today and after swapping them out I set the audio delay to negative, set the TV audio to passthrough, and the problem is completely solved. We will be returning the Alto 8i. Unfortunately, every story I have read here or on reddit basically ends in returning the 8i because this is a known problem that cannot be fixed.
I hope this helps. Highly recommend checking out the Vizio V-series soundbars instead!