Is the clock consistently not playing alarm sound?
Is the clock app up to date?
Have you taken an update of the app recently?
The phone may just need a reboot and/or cache cleared. Or the app may need updated. Conversely, if it was updated recently, that could be the issue. Assuming it's a system app, you can uninstall updates to revert the app to the version originally installed on your Pixel.
To get to the app's settings screen: Settings>Apps & notifications>See all apps>Choose Clock. Tap the menu icon top right and there is the option to uninstall updates. Select Storage & cache to.. clear the cache.
If the problem isn't constant, it's gonna be a pita to troubleshoot. You could also disable the app and download a third party one, but I'd be concerned about reliability, considering how aggressive Android is with memory management.