My iMac built-in microphone doesn't work! Help please!!

I bought a used iMac from a seller on ebay. Everything works perfect, iSight camera is excellent. I tried recording a video and it worked, when I played it back it had no sound at all. I already tried going to system preferences and changing it to built-in microphone. I also reset the PRAM. I don't know what else to do!! 😟

It has the version 10.6.8 installed, I don't know if that has anything to do with the mic not working. I hope you can help!