I've just got into the various floatplanes for FSX (and P3D) and I noticed that in some aircraft, while moored, there are sounds of the water laping etc. whereas in others there's none of these sounds. Is this something I can add to aircraft at all?

When we did the Republic Seabee the water lapping sound was a recording that looped when the anchor was lowered, I suspect that was the same for other producers ,  While I have done a number of float planes the Seabee was the only one that could float without additional equipment , pop the front hatch and drop your fishing rod in the lake , the cabin had folding seats , you could go camping in this baby .

 

The one amphibious plane left on my to do list is The Explorer’s Air Yacht  a Sikorsky S-38,  a must !

 

Just about the coolest toy the moguls of the day could buy , This replica was featured in the film " The Aviator "  

Best CJ

 

In the sound.cfg try looking for something that refers to the water lapping sound.  Sometime developers use wind_sound or similar.  You may be able to copy that and the corresponding .wav to the aircraft that lacks the sound.

This is one example of how Milton Shupe did it in his Mallard's sound.cfg/sound folder.

[GROUND_ROLL.2]
filename=Grummanwater
flags=8
link=GROUND_ROLL3

There was a .wav file added to the sound folder called Grummanwater.

This might help maybe.  Josh

http://www.fs2000.org/2017/09/17/fsxp3d-mccoy-water-landing-effect-v4/

Thanks very much for all your replies: I did actually have a go at finding a wav file from another aircraft which had water lapping sounds and modified the sound.cfg as per your example Charlie - and it worked! Fantastic. Thanks very much!