Compaq presario sound card speakers quit working when add new extenal speakers i

right click the speaker icon in the lower right hand of the desktop, do you see the volume control? double click the same icon, you should see the audio mixer...
but in case there is no volume at all...

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.