to check bios, you will need to reboot. on the first screen you see, it should tell you what key to press to enter bios (often delete, or F1, F2). press that key a few times, until bios opens up. you will have to use the arrow keys to navigate. there will be a page named containing onboard periphirals, including the sound.
if you have the manual for your motherboard, it will contain a list of all bios settings and where they are. you could also look it up online, if you have trouble finding it. or tell me specifics of your machine (make and model, etc.) and I can try and find it.
make sure onboard sound is enabled. if not, save and exit, by pressing the F10 key.
if you accidentally change any other settings, exit without saving changes, and everything will be restored to what they were.