Okay, managed to get hydrophone sounds from the imported ships!
Unit type 104 (coastal vessel) seems to be problematic, it won't show up in audio contacts at all. So no way of knowing if there is a 4000 ton ship passing over you, sonar man won't hear it. Changing type 104 to 102 seemed to do the trick (plus some other tweaking). I'm not yet sure if there is a way around this.
Trevally, what would happen in the campaign if we changed the 104 coastal ships to regular cargo 102? Would it mess something up?