Since i reinstalled my laptop a couple of weeks ago the sound of many applications randomly got reduced requiring me to go into the sound mixer to restore the levels. I noticed this most often when i plugged in my iPhone into the computer.
Since i initially thought this was related to my iPhone in some way i googled around and found a suggested solution by kleptomaniacist on the apple forums.
To stop this you need to go to:
Control Panel -> Sound -> Communication Tab -> and select Do Nothing.
Windows 7 implemented this so that when it detected communication on your computer it reduces the sound of other applications, which while useful if you are communicating i have found it irritating when it falsely triggers reducing the sound of music. This most likely only affects windows 7+ computers.