Ok, I think the issue is not related to the RGB Dual HAT iteslf (and not to the ipac USB). I did the following:
1 - Burn a new image with Recalbox 8.1.1
2 - Disconnect from the Rpi the ipac USB and the RGB Dual HAT
3 - Connect to the Rpi just a HDMI cable.
4 - Boot normally
5 - Go to SOUND SETTINGS->OUTPU DEVICE and change from "HDMI/Displat Port . HDMI" (default value, since HDMI cable is connected) to "ANALOG OUPUT - HEADPHONES". Sound starts to play on the jack.
6 - Now go to ADVANCED SETTINGS -> RECALBOX CRT -> CRT ADAPTER and select "RECALBOX RGB DUAL" to tell the OS you want to use the HAT (even it is not yet connected, the menu shows the option)
7 - Reboot. The system will reboot twice (I undertand due to some resoluton change). Onm the first reboot there is stil some video audio.
8 - Now the recalbox menus are shown on the HDMI screen (in CRT resolution 240p), and no sound is coming from the jack.
So it seems that the OS switch to "RECALBOX RGB DUAL" in step #6, for some reason, interfers with the audio jack.
Do a Factory Reset to set delete the CRT ADAPTER settings, so HDMI is back, and sound is back.
I continue testing with other CRT adapter settings, but it would seem an issue on the OS not linked to any ipac or actual RGB HAT.
Toni