Optical analog audio intercom, LED based...

audioguru said:

The IR amplitude is modulated by the carrier frequency of the PWM. Then the PWM Frequency Modulates the carrier frequency.

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I see no reason for such a convoluted modulation scheme.
Why not just directly turn the LED on and off with the PWM digital signal?

PWM produces AM, not FM. It brightens and dims the IR LED. AM produces all kinds of interference caused by anything that produces IR so the sounds will be full of "Rice Krispies" noises (snap, crackle and pop).
FM does not produce those noises if the detector is designed properly.

I think you two are talking past each other.

audioguru said:

PWM produces AM, not FM. It brightens and dims the IR LED.
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There seems to be a fundamental misunderstanding here.
It's true that an LED can be made to vary its brightness using PWM but that's because your eye is doing the demodulation (averaging of the signal) of the higher frequency PWM digital signal.
The duty-cycle change of the PWM signal is what carries the modulation, not any amplitude change.

The signal itself has no amplitude variations and is largely insensitive to any amplitude variations caused by external noise sources, similar to FM. The received digital signal is squared by a comparator or logic gate to remove any amplitude variations before it is low-pass filtered to recover the modulated signal.

You are correct. PWM can modulate the IR beam and a simple lowpass filter demodulates it into clear audio.
But in Google there are many circuits that use a CD4046 PLL to FM modulate and FM demodulate an ultrasonic carrier frequency that I was thinking about.

Never done audio over optic, but I have done audio thru the ground, 1/2 a 4046 (vco part) makes a good fm generator, and another 4046 as a fm decoder at the rx end, with optics you'll be able to use a wide bandwidth.
If you really want blue light fine, white or infra red would give more light o/p though, a laser is probably easiest to aim.
If you put a 1w led on the side of a hill somewhere you'd be surprised how far it goes, miles, aligment is the tricky bit.