This problem is more than likely caused by a program of some sort that you use for music or video. An example: I use MusicMatch for mp3's. When you turn the volume control up in MM it turns up the wav volume control in the volume control applet. If you turn it up, then close the program, the next time you open MM it'll bump the wav control up to where it was when you last closed it. So, if you turn down the wav control in the volume control applet and then reopen MM, you'll find that the wav control is once again turned up. The obvious work around to avoid this is to set the volume back to where it was before you exit the program. You'll find that some of these programs will control the volume using the wav control and others will use the play control itself to turn up the volume. I've come across a few that will give your choice in the options of the program.
I'd be willing to bet that what's causing your midi control to move is your web page. I just had a look at it and while viewing it, I took a look at the midi volume control. It was set right in the middle. I then closed your page, turned the midi control up and then went back to your page. Guess what. The midi control was once again set back to the middle.