My setup is a CCwGTV connected to a TCL 55R617 and from there to a Klipsch soundbar via optical. The first half second of the Google assistant's speech is always cut off (e.g., if you ask it what time it is, you'll hear "irty five pm" instead of 7:35 PM), you don't hear why system sounds (likely entirely clipped), and many YouTube videos have random half second audio cutouts throughout the video. Bizarrely it's not all videos and it only seems to happen in YouTube (i.e., no issues in Netflix, Prime Video, etc). I'm not sure if the two issues are related but I don't have these issues with YouTube on other platforms.
Thanks for reaching out. Please try to clear the cache and data of the YouTube app. Reinstalling the app and rebooting your Chromecast with Google TV and soundbar might also help. If the issue persists, please reset your Chromecast.
Note: If the issue is isolated when using the YouTube app, please contact YouTube for further help.