Double click the speaker icon in the lower right hand of the desktop, this will bring up the mixer, or single click to bring up the volume control.
But in case the volume is missing, corrupted, or the audio is otherwise non-functional:
you check whichever audio was installed:
click start, control panel, add remove programs, look at the list...
which audio software is shown on the list? delete it, restart the computer.
now visit the motherboard manufacturer or computer manufacturer's website
download the latest audio drivers, save on the desktop...
turn the antivirus off,
if you have multiple security programs installed, turn them all off
(your driver may not install if security systems are turned on)
now load your new audio driver, restart, turn the antivirus back on.