I do not believe this is a hardware issue yet I’ am still hanging wither a KDE issue. I did an emerge world after a fresh install of CLD 13.4. The box is now 13.6 but the sound does not work. The hardware is there, in the mixer, alsa, phonon settings, etc it is there. No sound comes out and no error comes up. The mixer control does say there is no playback streams when there is. The only odd thing I noticed was that with the new release KDE asked me to forget the old drivers, Intel HDA 97xx for the new ones.
Run alsamixer and press F6. How many sound cards shown? Which one is the default? May be you have usb-webcam(It often has its own microphone and defined as sound card)?