Just now testing Channels DVR to replace Plex DVR (plex's guide implementation on the firetv is absolutely horrible. You can't scroll without it glitching out). Starting to notice audio/video sync issues tho on live TV. I tried playing the same channel on my FireTV in both the HDHomeRun software and Plex and the sync issue doesn't seem to be there, only in the Channels app.
Which FireTV model?
Firetv gen 2, gen 3 and 2 firetv sticks gen 2
I, too, have started to notice A/V sync issues whether live or DVR recordings. Oddly, it looks to me like the audio is behind the video by a fraction of a second. (I say oddly, because usually it's the audio that gets ahead of the video because the TV introduces some additional latency with all its fancy video DSP).
My playback device is a Fire Stick 4K purchased in December 2018.
There seem to be so many variables to getting the A/V to sync (not just the playback device, but the TV, the soundbar, etc.) I wish the Channels app just had some kind of control where I could manually move a slider to get the audio/video to line up.