First, Try plugging in some external speakers or headphones. If the sound is good, then it is definatly a problem with the speakers on your computer. If its a Laptop you are using then there is nothing you can do really, except get a new computer or use headphones when you need sound
If its a Desktop PC you are using then just get some more speakers
However if the sound is bad on the headphones as well and you are using Windows Media Player try Right Clicking on the top bar and then clicking on View - Enhancements - Graphics Equalizer
This will show some bars that go up and down. These should all be centered if not, reset them.
This is all I can suggest really so I hope it helps