I've had this problem several times in different Minecraft versions, and on different computers and also with two different kinds of headphones.

The headsets I've been using is 'Sony DR-GA500' and right now 'SteelSeries Siberia V2 Black/Gold'.

This sound bug appears every time i open up Minecraft and going in to ANY menu except the start menu. The sound level diverge in every different menu, so for example is the sound pitch level when I go into the multiplayer menu higher than the single player menu and so on. This sound pitch also appear when I am pressing any menu buttons, e.g. when I click "Join Server", it's almost like I hear Minecraft "working".

To reproduce this it's a simple matter of:
1. Open Minecraft.
2. Click on any menu.
3. Listen.

I've heard that many other have this problem too, and some people doesn't have it at all. So it's pretty weird.

I've also provided you with my DxDiag file.