The reason for this problem is that the field has a widget_type audiofield_widget and a widgetmodule audiofield. So I naturally tried to upgrade the audiofield module, but this does not solve the problem. CCK migration simply refuses to migrate these fields. Dunno why exactly. (Instead of Missing field module: 'filefield'. This field cannot be migrated. a better error message would be: Won't do audiofields. Don't fucking like them. Sue me.)
Manually change the widget_type to filefield_widget and the widget_module to filefield. You can do this via phpmyadmin for instance. Or to do this for the multiple audiofield we've defined, run this query
set widget_type = 'filefield_widget', widget_module = 'filefield'
where widget_module = 'audiofield';
If you run it directly use the table name with prefix. The above you can run in Devel.
I will post here lateron how to adress the presentation in the final upgraded site.