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It's a very crude circuit and has no protection or regulation of laser current so it could easily cause permanent damage to the laser diode.
It is an emitter follower circuit with the laser diode as load. The current it passes depends on the voltage between the base and emitter of the transistor. The emitter is held at fairly constant voltage by the laser and the base is held at 5V from the regulator with the audio from the amplifier added to it. The audio voltage therefore changes the base-emitter voltage and laser current.
The sound quality is likely to be poor and may vary from one construction to another. The 10 Ohm resistor should NEVER be there and the one marked 0-? SHOULD always be there. I would guess this wasn't designed by anyone with audio or electronics experience.
Since the 10 ohm is in series with the battery, as the Laser current varies, it will cause the Led brightness to be slightly modulated.
Altogether a very crude and primitive circuit.
EDIT: what exactly are the photodetectors on the right side? LDRs? Photodiodes? Phototransistors? Photovoltaic cells? Integrated receivers?
Each one of them requires a different biasing and loading circuit, which is not shown.
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