The mixer you're looking for probably wasn't a part of Windows, but a part of your sound cards program.

It appeared as an icon on your task bar. If you had a Creative SoundBlaster card, it did include a bit more control.

Since you have a built in Realtek, you may need to find the Realtek icon and create a shortcut on your desktop. That's what I did.

It will give you a bit more control, but not like the old SoundBlaster mixer.