Hi,
Apparently, the MD pin of the core has to be connected to the SC pin of the audio module like described here:
http://www.midibox.org/dokuwiki/doku.php?id=midibox_sid_faq#how_do_i_connect_the_sid-module_to_the_core-module
I uploaded the V2 hex file and now the Sid is alive and I have sound at the output.
Thank you for you help!
Nick