the main problem is because your "realtek audio driver" has been updated automatically to a newer version which the Sidetone function has been removed.
If you could think of a way to uninstall the current audio driver and go back to use an older one, you could have the sidetone back.
But I haven't got a stable solution to solve it, because I think my computer kept updating whenever possible. I had to keep reinstalling to an older one to have the Sidetone back.
it's a very good function I don't know why they removed it.