Now this is more like a VCO with a very simple one-transistor amplifier: -

[![enter image description here][1]][1]

It's a VCO because it has an input (Vin).

Picture highly likely to have come from the same [French-speaking site][2] as the original.

Try putting your speaker in series with the collector of the NPN transistor and make the emitter resistor fixed at about 1 kohm or greater. It'll be a little louder.

If not loud enough then use the ubiquitous LM386 audio amplifier connected to the pot's wiper as shown in the picture above.

  [1]: https://i.sstatic.net/goQ2d.gif
  [2]: https://www.sonelec-musique.com/electronique_realisations_vco_003.html