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I've been having a problem on my new laptop recently which is familiar but baffling the usual fixes. I'm running Windows 8 with an onboard Realtek soundcard. It's similar to the one on my older computer running Windows 7.
The problem is, when I'm in Skype or Mumble, Windows changes the sound output to lower everything else automatically. I've disabled the communications sound change option on the communications tab within sound devices and checked all the applications settings to insure that they are not responsible. They aren't, and I noticed something else. When I'm in the sound properties dialog, and I switch to the microphone tab, the same audio output reduction occurs. This seems to say to me that the microphone must be responsible in some way, but seeing as I uninstalled all the drivers and installed windows drivers instead, I'm confused as to why this would be occurring.
EDIT: I just tried disabling the built in microphone and the sound no longer get changed. More confused now? As soon as I turn it back on, the sound gets dropped again.
Incidentally, the fix for this on windows 7 was this question: Windows 7 lowers applications' volume automatically I've got my computer set that way and it doesn't work.