Why the tone dies in the dark
Experiment 5 swaps a resistor for a photocell and the pitch follows the light. Cover the cell and the tone stops. Here is what starves.
Experiment 4 is the tone: two transistors taking turns, each one switching the other off through a 0.1 µF capacitor. The capacitor charges through a 10 kΩ resistor until the next transistor turns on, then the pair flips. Because the capacitors are small it flips about seven hundred times a second, and the speaker on Q2's collector turns that square wave into a note.
Experiment 5 is the same circuit with one change. The 10 kΩ feeding Q2's base is pulled out and a CdS photocell goes in its place.
A CdS cell is a resistor that changes with light: about 1 kΩ in bright light, hundreds of kΩ in the dark. Now the light level sets how fast Q2's timing capacitor charges. More light, fewer ohms, a faster charge, a higher pitch. Slide the little moon-to-sun slider under the cell and the note bends with it. That is a sensor. Something physical, turned into ohms.
What happens in the dark
Drag the slider all the way to the moon and the tone stops. Nothing is broken. The cell has climbed past 100 kΩ, and so little base current gets through it that Q2 can never turn on hard enough to flip the pair. Q1 stays on, Q2 stays off, the capacitor sits there charged, and the oscillator stalls. The speaker gets a steady voltage instead of a square wave, and a steady voltage makes no sound.
Slide the light back up and the base current returns, Q2 switches, and the whole thing sings again.
Try it on the bench: open experiment 5, turn the speaker on, and work the slider. Hover the photocell while you do it and watch the ohms go by.