Please be a little more specific as to the kind of HP you have. Make, Model#, age.
Assuming you have a laptop. There are function keys (Fn + 'a key') will either increase or another set will decrease the sound. Take a look on the keyboard. Press the Fn key plus the increase key and hold it down until the level is correct. The other thing I've seen happen is with an update or SP-1, the sound gets mutted. Check in the system tray to confirm the speaker is not mutted, or go to the control panel, Sound, Speakers, Levels and make sure the speakers are turned on and the levels are mid way +-.