l. Configuration issue. Reinstall the audio driver to reset configuration to normal.

Open the Device Manager. (Press the Windows key + the R key, then type devmgmt.msc into the Run box, then click "Okay".  Or you can type devmgmt.msc into the Windows search box.)
Next, expand "Sound, video & game controllers".
Right click on Realtek High Definition Audio to open the context menu.
Click Uninstall.
Do NOT check the box "Delete the driver software for this device". Leave the box empty.
Click OK.
Windows will uninstall the audio driver. When you restart the laptop Windows will re-assemble the driver from the files that you did not delete, but the driver's configuration will be back to its default settings.

ll. Driver issue. Test by switching from the vendor's audio driver to the Windows native driver. If it works with the native driver then something buggy in the vendor's driver.

1. Open the Device Manager (type devmgmt.msc in the search box and hit Enter).
2. Expand the "Sound, video & game controllers" and right click on "Realtek High Definition Audio".
3. Select to "Update Driver Software".
4. Click on "Browse my computer for driver software".
5. Click "Let me pick from a list of drivers on my computer".
6. Put a check in the box "Show compatible hardware" if not already checked.
7. In the list of devices, click "High Definition Audio" (the native driver).
8. Click "Next".
9. On the Update Driver Warning box, click "Yes" (install the driver).
10. Restart the laptop if prompted. If not prompted, then no need to restart.
[To get back to the Realtek driver,  do it again but reverse the names in steps 1 and 6.]

lll. If still no luck then probably a hardware issue. Contact tech support for warranty support.

Thanks for extremely well written instructions and suggestions.  Unfortunately no luck, so will contact tech support as you suggest.

Many thanks.