[SOLVED] Sound level too low on nternet videos

I recently purchased a new Dell Inspiron 570 with Windows 7 x64 already installed to replace my old XP system which died. I moved my old speakers (stereo w/subwoofer) to my new computer. I am using the computer's integrated sound card. The computer tells me that I have the speakers properly installed (all sliders at 100% & test tones work OK.) Also audio CD's play just fine. Video music DVD's play OK; however Teaching Company DVD's behave strangely: The music intro is OK but the professor's lecture is almost inaudable. Videos on the internet behave similarly: voice content is almost inaudable
I have selected the "do nothing" option in the control panel sounds app. No help there.

Any help would be greatly appreciiated.

I purchased ($31.99) and installed new Diamond Xtreme Sound 7.1 card. It fixed my problem