I assume you're using Windows. Look for the little speaker icon next to the
clock display on the bottom right corner of the desktop.

Double-click on that and the volume controls will open. Go to the Options menu,
select Properties and turn on all the volume controls you can find in there. Click
the Ok button and start playing with the sliders until you get what you want.
The "Wave/MP3" slider is most likely the one you want.

While you're there, go back to Options and click on "Advanced Controls" if you
have it. That will give you some extra "Advanced" buttons for a few of the
sliders. You might like what you find in those controls too. ;]

Oh, and by the way. If your speakers have their own volume controls make
sure they're not turned all the way up. About halfway will give you much
cleaner sound because most will give you sume buzz and hissing at full volume.