In Windows 7, you can set the master volume independently of the volume of individual applications. Right-click the speaker icon in the notification center (near the clock) and select "Open Volume Mixer." The leftmost pane is the master volume. To the right are individual applications. Make sure you turn the volume up for the application and the master volume!
You could also right-click the speaker icon and select "Playback Devices." Select your speakers and click "Properties." Try changing the settings in the "Enhancements" tab to see if that helps!