(This has a solution - at least an interim solution - at the end, this is sharing a weird issue)
My wife has a Probook that's a few months old. A few weeks ago the sound went funny - instead of being clear you can't make out what it's saying. It's scratchy, high pitched, and weird. Headphones work fine. We assumed hardware problem.
We called HP support, who did the usual checks for BIOS and drivers. An on-site visit was set up. The guy turned up, uninstalled the drivers, let windows reinstall them, and it worked again. Yay! He told me HP doesn't support Windows, just the hardware.
But when my wife logged in this evening the sound didn't work. I uninstalled drivers again, restarted, ran windows update. Yay! It works again. Restarted, it's failed again.
I have narrowed it down to something to do with Realtek driver by uninstalling sound drivers one by one. I uninstall the device in Device Manager, reboot, run Windows Update, it installed driver 6.0.8838.1, and sound works, yay! Then I reboot and it's failed again. Driver version is exactly the same in device manager before the reboot. Headphones always work.
I've run the HP Support Assistant to check for updated drivers. It found 11 updates, but none appeared to have anything to do with sound. So I downloaded the latest driver manually from the HP website. Odd that it isn't detected by Windows Update or HP Support assistant. Driver version has updated to 6.0.8996.1 dated 28/7/2020. A couple of restarts later and running Windows update everything seems fine... for now.
So if your HP Probook has sound issues, try downloading the driver manually.