I ran into an unusual problem though : the circuit seemed to work only during half of the cycle of the signal, outputting no sound on the other half. Overall the sound level was very low. This did not happen previously on breadboard. I reflowed the solder and checked the layout with the schematic, everything fine.
I first thought a damaged capacitor was responsible, and changed all the electrolytics. I then measured all the 1u coupling caps, and de-soldered the other caps to check them. No change.
I then discovered the 4.5v voltage splitter behaved erratically, fluctuating quickly between 3v and 7v. The 9v input did not fluctuate. After checking the op amps and transistors, I discovered this behaviour appeared only when the input buffer transistor was connected (I checked with various 2SK117, all in working condition).
If this buffer is bypassed there is no problem, the half-voltage rail is stable at 4.5v. So I connected the input to where the source of the transistor should be on the layout and replaced the 10k source resistor by a 1M pulldown resistor. Everything works now fine.
I would be curious to know what the reason of the problem could be there, as I cannot explain it. Any thoughts ?