Hi
So: voice 3 was silent but the internal red LED was on when I pressed a key.
1) Swapped the 2056 chip with another one from a working voice: no change;
2) Swapped the 2044 chip with another one from a working voice: no change;
3) Followed Florian ideas for checking signal at different pins with oscilloscope: signal was here
4) Checked the signal at TP1 Sig out: no signal.
5) On the schematic, just before the TP1 there's a chip and resistor array, checked the resistor array: no signal at the voice 3 pin.
6) Before the array is the voice 3 out on pin 1 of IC33 (4558 chip): signal was good !
7) Checked the continuity: obviously it was dead. It was just this !
(－‸ლ) I missed it when I checked the board ...