Just thought of something..
When listening to the ship propshaft revs, you can count the revs per Knot (Not with RFB - they've manipulated this / Not sure about TMO).
At the lower end 3 Knots or less you hear nothing (stock game) and up at the ships top end there is not much change.
Now this lot is dependent of the ship power and rev ratings in the ship data file,and it follows a forward slanted S-Curve (hysteresis curve). If you change the power ratings you can effectively blank out the sound.
This might be the problem as I can hear the DDs but not the merchants, and it make me think that it's a ship power/rev thing
I did this long time ago when putting a speed-rev chart together - time to redo this
