Why is my loudspeaker not working? From Make: Electronics

The frequency of oscillation is determined by the 470 kΩ resistors and 3.3 μF capacitors and will be roughly \$ \frac 1 {RC} = \frac 1 {470k \times 3.3\mu} = 0.64 \ \text{Hz}\$. i.e., less than one pulse per second. The circuit has been optimised for the LED rather than the speaker.

Modify the circuit so that the frequency is 100 to 1000 times higher and you should be delighted. Note that if you put the LED back in you won't be able to see it blink (due to your persistence of vision) although if you can wave the breadboard back and forth you may be able to see a "dashed" line of light.

The oscillator you have is a multi-vibrator. It outputs a square wave. A square wave of low frequency sounds like a series of clicks when fed to a loud-speaker.

At a higher frequency, as square wave sounds like a buzz. At even higher frequencies, it begins to have an "electronic music" type of sound.