The speaker was rusty, marked with "X" sign by somebody and dead:

According to the schematic of issue 2, the speaker is driven directly by the ULA output through two diodes to reduce the ULA output voltage, as I understand it:

Some sources said the speaker impedance should be 40 ohms, others 200 ohms. In any case, I couldn't find a replacement speaker other than 8 ohms. So I decided to use it in series with a 39 ohm resistor and add an S9013 transistor as an emitter follower, similar to the circuit of issue 3:

The new speaker and additional elements are a perfect fit:

The resulting volume level is completely good.