Could someone explain why nearly all amplifiers I see only amplify one half of the wave? I am seeing these optocouplers as only being able to amplify the positi

Could someone explain why nearly all amplifiers I see only amplify one half of the wave? I am seeing these optocouplers as only being able to amplify the positive side of the cycle and ignoring the negative. In my mind I see the sound pressure as a positive and a negative where each side could be very different but ultimately adding up to zero over time. Is it just that if you make a positive pressure wave the negative is right after and does not matter? Class A amplifiers seem to take both sides into account. I am expecting to see some sort of H-Bridge configuration but never really see it. The most I see are positive and negative rails. What am I missing here?