Half Volume - Why?

Sometimes, usually while in a video game. Most recently L4D2. My sound output to my headphones just all of a sudden gets quiet. Approx. 50% volume. I checked all volume controls, they were fine. I unplugged and replugged headphones. I sent it to speakers. The only thing that fixes it is a full system restart.

Why is this happening? How can I fix it?


And while on the subject. I want to plug 2 headphones into the multiple jacks on my computer and have them play sound at the same time. Is there a program that allows this since Windows 7 does not by default allow such.

This might be a fairly inexpensive answer to your 2nd question:
3.5mm STEREO JACK Y-SPLITTER ADAPTER PART#05004
:)
I'm not advertising this particular online store, it was the first result with a decent picture

I regards to your first question, do you by any chance run a VOIP system through your computer?

I have that setting set to Do nothing. But I use ventrilo if that counts as a VOIP program.

Ventrilo is most definitely a VOIP program :)
Is it running in the background while you're playing?
If so, could it be possible that you have a key binding set within Ventrilo that mutes the sound to half volume?
Just a guess mate, I've never used it, but read it has "hotkeys" for various functions.