Electronic – How to find linear LEDs (light power(mW) vs current)

If somewhat more than minimum cost is acceptable you could implement a buck converter per LED. These could be very crude and low component count and not a vast price. Each converter would be able to set a current that was proportional to a PWM or analog input. Probably under $1 per channel.

Your 3 x Power supplies plus resistors idea is acceptable BUT can be improved in performance (with increased cost) by making it 3 x power supplies plus constant current sources. The constant current sources are not much different from the buck converters above but with no inductor.

Example. Imagine that your lowest Vf LED (probably red) has a Vf range of 2.0 to 2.5V.
You will need at least 2.5V supply plus headroom. Say 2.7 V.
Minimum efficincy will be when Vf = 2V = 2/2.7 = 74% BUT this is worst case Vf and in many cases the Vf range is lower.
eg say VF = 2.2 - 2.4. Vsupply = 2.6. Zmin = 2.2/2.6 = 85%. Boost supply to 2.8V for more headroom. Zmin = 2.2/2.8 = 78%
Again, this is worst case, so acceptable.

A buck converter will improve on the above.

One solution of zillions. Rsc not needed. Rather than sensing output voltage, LED current is sensed. This C had Vref = 1.25V = far too hight. It can be reduced to 0.6V effective using a diode but still too high. More modern ICs has a lower Vref OR a 1/4 IC section can be used per channel. PWM per channel then becomes 1 x SOIC MC34063 + 1/4 LM324 + a few resistors and a catch diode per channel. May be cheaper than more modern IC.

This circuit, from sound.westhost.com may be suitable.
I independently arrived at a similar version - works reasonably well. This is voltage feedback. Can be made current feedback. Vsense here = Vbe of Q1. This can be reducued with care.

There is already a solution to converting stereo audio to a digital optical signal and back again that's widely used and readily available: TOSLINK. It basically is a simple conversion from the electrical S/PDIF signal to an optical format. There are many commercial chips that can convert directly between analog audio and S/PDIF (in both directions).

The only difference is that TOSLINK normally uses inexpensive plastic fiber to couple the light between the transmitter and the receiver, and you want to do a "free space" transmission. Therefore, you are going to have to focus on the optical part of the link, dealing with highly-variable signal levels and lots of potential interference.

White phosphor-based LEDs tend to have poor frequency response, mainly because of the slow decay of the phosphor's glow, although if you read datasheets carefully enough, you can find some with fast phosphors.

But you could use the slower LEDs if your receiver includes an optical filter that passes the blue light directly from the LED and ignores the yellow light from the phosphor. This might be a good idea in terms of rejecting interference anyway.