Sound card problem: static/crackling and no sound

I've had a late 2010 Macbook Air for over 3 years (February 2011). About a week or two ago I started hearing a strange popping noise every now and then while I was using headphones. I thought it was a problem with the headphones but then discovered the same thing when I used different external speakers. Almost simultaneously the internal speakers started periodically having no sound at all, even though it wasn't on mute. This was previously fixed by restarting the computer, but now the problem is permanent. The popping noise when using headphones has also rapidly worsened, and now crackles constantly and sometimes there is even very loud white noise. This occurs with any program on my computer - web browsers, spotify, itunes etc. As it's over three years old, I no longer have AppleCare. I'm not too keen on taking it into the Genius Bar unless I have some idea of how much it will cost beforehand, as I also need to replace my battery soon, which is going to be 129 euros. Does anyone know how to fix this problem, and if not, how much does it usually cost to replace the sound card? I'm using Mavericks but the problem started some time after I installed it.

After doing some googling to try to fix it I've tried:

- Turning it off and on again

- Installing AntiPop

- PRAM reset

- SMC reset

- Disabling and reinstalling Adobe on google chrome

- Resetting the sound card with terminal