MPlayer has two options for increasing volume above the sound card's maximum setting. Adjusting MPlayer's software volume, or using an audio filter to amplify the audio. I had a problem where my DVD's would play too quiet, even when the volume was maxed out. Using this I was able to adjust the volume so the DVD would play at the same volume as any other video and i wouldn't have to readjust it each time a changed between them. The Audio filter method tends to work better