Try plugging a set of headphones in to see if they work.
As for installing a new card it is easier than you think. Just follow a couple of rules and you will be OK. 1) turn off your computer and leave the plug in the wall, with the power OFF. 2) Earth yourself by rubbing your hands over the tower casing. 3) open the tower up and remove the old card. 4) DO NOT touch any of the gold coloured parts of the new card. 5) make sure the card is seated correctly (you should hear a click).
The reason for leaving the plug in the wall is to allow you to be earthed. If you are not earthed you may cause damage to the internal parts of the computer through static.