The only Posky plane I see on the page is the A340, so I'll assume thats what you mean.  I just checked and the sound it uses is the default Boeing 737 sounds from FS2004.  I never really noticed that they sounded like a prop plane, but they aren't that great of sounds.

I would suggest simply downloading a different set of sounds.  There is a sound library here at SimV.  I suppose I would just pick one and try it, if you don't like it try a different one.  I would back up the file found in the plane's sound file first though.  Anyway, if you want to look for a different sound file, check HERE

thanks for info