I did a test by also enabling the TV speaker.
The TV itself doesn't have the delay.
I changed digital output to raw and audio output from variable to constant.
The delay is better now but still present. At least the gun fire sound starts before the recoil finishes now.
Strange how nothing else exhibits this delay even on the same CCU.
I was going to make a new thread and this popped up so I'm hijacking.... I've had an audio lag in Destiny from day 1 and it's still happening. I've tried two different CCU's, two different TVs (One an LG C8 and the other an older Samsung 1080p). When firing a gun in Destiny, the audio arrives a split second after the gun fires. Think of a handgun, the gunshot sounds when the gun is at the top of it's recoil, and not as it fires, as on every other platform, and indeed Stadia running on Chrome. I'm saying it's only Destiny, as that's the most noticable, but it may occur in other games. Testing the delay on YouTube on the CCU shows no such latency. It's just Stadia running on the CCU. LG is set on Game Mode for both sound and picture.
There is more than one problem here I guess.
First of all, Destiny 2 has some audio delay on certain effects (e.g. jumping and hitting the ground again), while other parts of the audio can be synced if you play it on a PC via Chrome-browser in stereo sound. So there is an in game sound issue that can't be fixed by the user.
The 2nd problem are the audio sync issues on Chromecast Ultra that many people experience. The CCU will most likely send uncompressed 5.1 audio (PCM) to your TV. If your TV can process this at all, it can only forward/passthrough it to a soundbar/audio system via ARC HDMI but not via optical out. If you're useing optical output the TV first has to convert the 5.1 audio to stereo PCM before outputting it (regardless of TV model). This means it needs time for processing etc. The soundbar/audio system itself then probably has to upscale/convert the audio again to match its own channel layout.
All this means additional audio processing on top of existing in game audio delay which results in a very disappointing experience on a TV/CCU setup like this.
It's better and less noticable in other games but the main problem on Chromecast Ultra can't really be fixed yet unless you get a setup to match all of Stadia's (unwritten) requirements for 5.1 audio.