Did you just upgrade/install Windows 7 on that PC? If yes, its likely that the default drivers
that windows installed lack the functionality to use those buttons. My Acer Aspire 5920 was like that initially. Installing the Acer supplied driver
got everything working. Go the HP site and have a look at the driver downloads offered for you model. Start with the sound and WIFI drivers. If that doesn't work look for any utilities offered for control of media keys or something similar. My audio/media keys were actually part of my touch pad and didn't work until I installed Acers custom synaptics driver.