I'd still like to know Mike's thoughts on what would happen without smoothing
Sorry I don't have internet access at the weekends so just seen your comments.
My thoughts are that the LDR would not pick up the pwm component of the signal as the response time of the sensor would be too low. This is based on experiments I did too many years ago to contemplate (I was still at school) where I tried to put music down a modulated light beam into an LDR. The result was that nothing came out at all.
However, until you try it you won't know. If there is a signal transfer then there is an options to increase the PWM frequency above the audio rate.