[mc68k]

The external audio port is working, so the sound card has no problem definitively.

Meanwhile I've found the internal speaker issue: the black wire had a cold solder joint on the speaker itself. More precisely there was absolutely no solder under the isolation tube.

All good now.

[mc68k]

After extensive testing I can confirm that the 070-1367-012 SGI Version 5.3 Rev E IP22 Jan 29, 1996 PROM image works perfectly in R4400 IP22 Indigo 2.