The laser is Pulse-Position-Modulated with a high frequency. But the receiver is not tuned to the high frequency and doesn't filter out buzzing from street lights. The receiver is too simple, it is just an audio amplifier.
The receiver already has a photo-transistor as a photo-detector. A photo-transistor saturates when light shines on it. When it is saturated then it doesn't work.
A good system would FM modulate the high frequency of the laser. The receiver would use a photo-diode in the photovoltaic mode which does not saturate. Automatic gain control would control the gain of the amplifier. A tuned circuit would pass the modulated frequency and reject other interfering frequencies. The receiver circuit would have an FM detector to reject amplitude-modulated interference.
The receiver's photo-diode would still need to be shaded from light which would cover-up the modulated signal it receives.
I don't know how you are going to accurately aim the transmitter at the receiver, unless they are permanently installed.