After all, something did go wrong with this mail move from O 12.17 to OM 1: In OM's Advanced Preferences window, unchecking Notifications for new messages and Program event sounds was not persistent and had no effect.
I noticed that in OM's operaprefs.ini, the line Show Notification For New Messages=0 and the complete section [Sounds] was not present. I imported both missing elements from the operaprefs.ini of 12.17 and it seems to work.