Most probably you're using VoodooHDA.kext which currently is, as far as I know, the only way to get audio on the HP Mini's and Compaq's. However, that driver is still in its beta stage and one of the known issues is volume control; neither the normal function key combinations nor the menubar volume ctrl gui work.
But the Mini's speaker is pretty good and if I get my facts right, its got Altec Lansing under the hood which has no probs filling my room when I play songs on iTunes on it.
Try going to System Preferences > Sound. Adjust volume manually via the slider.
or System Preferences > VoodooHDA PrefPane (if you don't have it installed, you can download from the voodoo site) and adjust settings.
Hope this helps.