uhm...
well.. try reinstalling the sound driver.
if this doesnt help then your sound card is probably fucked up. overload i guess..
it
might be unhealthy for your motherboard if you keep using it.
if you cant solve the problem you should deactivate the onboard soundcard in the hardware manager.
if you need a new soundcard you first have to know what you want:
1. do you listen to high quality music (320 kpbs, FLAC rips or original CDs) on your PC or do you want to?
2. do you have really good speakers/headphones?
3. do you care about your sound quality?
if yes on all 3 i can recommend the Asus xonar essence STX soundcard to you. its pretty expensive but it sounds completly awesome. im using it for about a year now with logitech Z 2300 speakers and sennheiser hd 595 headphones.
if you dont want to pay so much cash then just buy some random soundcard. you have to decide yourself how much cash you want to spend but if you have cheap speakers and dont want to buy new ones anytime soon then it would be a waste to spend more than $30 for a soundcard.