If the volume on these files is low on other PCs as well, then you'll need to amplify the audio (using a program like audacity), remove the amplified noise, and use e.g. ffmpeg to create an avi file without volume, then bond it with the new, amplified wav file you made in audacity.
Ffmpeg is beautiful; http://linux.die.net/man/1/ffmpeg
Not ideal, but it can raise the volume.
If this only happens on youtube(?) for all youtube videos, it could be a hardware decoding error given that the graphics card is nvidia. Disable hardware decoding of youtube videos by right-clicking a youtube vid and de-select hardware encoding.
Other than that, need more info