a wired network connection and sound are on the motherboard.
a wired network connection from the motherboard is as good as an add in card, the cards are mainly just a way to cash in on people who think it may be better, when in reality they just tie up the PCI bus.
as for sound a dedicated sound card will be better than onboard but that depends a lot on the speakers you are using, if you use cheap low quality speakers you can chuck the best sound card in the world in your PC it will still sound bad. the only real advantage is some have built in amps for headphones and some have the ability to encode on the fly into Dolby or DTS even if the program/game you are using doesn't support dolby/DTS and isn't designed to be ran through a external surround sound system.
so to cut it short, a lan card isn't needed if you can use wired (it's better than wireless anyway) and it's only really worth getting a soundcard if you have good quality speakers or need to send the audio over optical/coaxial to a surround sound reciever.
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