When I reduce the volume down to its minimum in Windows 8, it doesn't mute, unlike Windows 7.
Is there any way to change this behavior back or no?
This is a manufacturing issue. Muting and setting volume are two different commands for any sound card (including television). The reason is that it is cheaper to build a card that goes to volume < 0.01 instead of a card that completely mutes (because of the electricity). The more you can reduce the volume the more expensive the sound card becomes.
In other words: When you have 0 in your mixer, it means 0 amplifying, it doesn't mean you do not get any sound, it just means that it is not amplified. And that is where the mute button comes in.
Update: The only thing you can do is giving out money and buy some better hardware.