What is wrong with my circuit and how can I fix it?
For loud sound you cannot drive the speaker through such a high impedance like 2k2. Also, your oscillator works at too low frequency (10Hz if pots are down) so it acts like a DC signal.
The circuit I attached below oscilates at about 1kHz and drives the speaker with higher power so the sound should be louder. Try to change C3 a little if necessary.