Don't use your sound card at the maximum volume level because of the high signal-to-noise ratio. For example, the SB Live's optimal value is 70% according to some tests.

Adjust the Master volume in the Windows Mixer to this value, and check it when you start a DVD software (for example, PowerDVD). Change both Master and Wave volume when you use its volume adjuster and you get lower sound level. Once done, you have to change back the Wave volume to the maximum.