Audio transformer for 12V light emitting circuit
Well, there is such thing "audio transformers". Here, take a look.

Your audio input must be amplified, it will not provide power enough for anything. And LED or laser will not just create PWM.

I would say, the whole idea is a little strange. Although i am not audio design engineer per say, i still wonder, what is it exactly you are trying to achieve? 

If you want isolated audio channel with pulse width modulated light, no problem. But why transformer? The receiver must be connected to a comparator, then to a gate driver and a MOSFET. All together would look much like a class D amplifier with optically isolated input.

If you want to transfer power by light, i would suggest separating power channel from data channel.