When troubleshooting sound issues there are two sources of sound generation inside:
The SAA is a little fragile, it does not like having it's outputs shorted or abused via bad connections on the SCART or Lightpen/Audio port which will render the chip silent.
No sound at all
Set a constant tone & BEEP running to get sound from both sources:
SOUND &2,255;&a,195;&11,4;&14,4;&1c,1 DO: BEEP 1,1: LOOP
Check at the SCART/DIN outputs, then move back to the OUTL, OUTR and MONX points in the schematic (between R42 and R43 around the SAA1099) and move backwards to determine where (and which) sound output is working.
Sound from SAA but not from ASIC
Set a constant BEEP running:
DO: BEEP 1,1: LOOP
Check output on ASIC pin 12, then R41, C19 towards OUTL, OUTR and MONX
Lack of output from the BEEP line could indicate an ASIC fault.
Sound from ASIC but not SAA
Check the 5V (pin 18) and GND (pin 9) connections to the chip, also the 8Mhz clock signal (pin8).
Put a meter on IREF (pin 6) and see what the voltage is, it should be around 2.5V, if it is nearly ground then the chip is dead.
Issue the following command to generate a constant tone from the SAA on both left and right channels:
Check the output on the OUTL and OUTR lines at R42 and R43 and then work back to pins 4&5 on the chip.
If still no sound then check the inbound data lines (pins 10-17) SOUND input pin (/CS pin 2) /WR (pin1) and A0 (pin 3)
/CS, /WR and A0 should toggle as register values and data values are sent, this can be tested with:
If no activity is seen on SOUND (/CS) this could indicate an issue with the ASIC or it's path to the SAA.
When removing the SAA, fit a 18 PIN DIL socket for the new chip.