My grandad has bought a computer and i tryed to show him how to play a dvd but no sound was playing. I found out there was no sound card? I dont actually know what this means but surely a computer should come with sound??

If there is no sound card. Probably it was disabled the sound and maybe there is no really sound card was installed. To make it short buy a PCI sound card, turn off your computer and install the card to PCI slot and turn on your computer and your computer will detect that there is a found new hardware. Ignore it and insert the cd installer and install the driver of your sound card. If it's ask you to restart just restart your computer to update its system. In case that there is a built in sound on your computer check your motherboard model and search for the driver from their official website and download and install, if you download the correct driver it will automatically installed your sound and your sound will be working. Have a nice day!