The correct term is 'cathode bypass' capacitor.
The capacitor is biased with a voltage larger than any audio signal it might pass. As a result, its distortion is quite low- rivaling film caps with no problem at all.
The reason electrolytic caps are reviled for coupling capacitors is because they have been mis-applied. For them to work properly, they must have enough DC voltage on them that the audio they are passing will not come anywhere near that DC voltage. In this way they will pass the audio signal quite well.