I also have a sound issue with the DP55WB. Sound works fine for a while, but when I try to load a fullscreen game like Borderlands or Left 4 Dead, the sound quits after a minute or two. Not sure what the problem is.
The low sound is probably the ear protection setting in the Realtek Control Panel.
If you look at this screenshot of the Realtek Control Panel: http://www.vistax64.com/attachments/drivers/7117d1223457080-no-2-1-option-realtek-hd-capture.jpg
It would be located left of the inactive Set Default Device radio button, and right of the speaker radio button. The ear protection button I'm talking about is not pictured in this screenshot, but it should be there for you when you have your speakers plugged in as you have described.
I have the same issue, when playing COD MW2, audio stops after a minute or 2, and I have to reboot my pc for solving this issue
also when using teamspeak 3, I have this issue
i'm using win 7 x64 as well