Today my Chromecast sound and picture were out of sync. To fix the Chromecast sound being out of sync with the picture, I disabled all Chrome extensions except the Chromecast extension, then went into the (advanced) settings of Google Chrome and reset the settings to their defaults.
This fixed the issue, but I’m not sure what sort of setting would cause Chromecast’s sound and picture to be out of sync. I imagine it was probably disabling my adblocker extension that fixed it 😕