I have only heard a plastic trombone for real, other plastic instruments I have only heard off Youtube video, and that includes a trumpet, however they have been manufactured in either moulding or extrusion machines where the bore will of course be nice and smooth. What the resultant sound would be like from a 3D printer I can only guess as it does depend on the finished condition of the bore, as it will influence the air flow and therefore the resultant sound.

However, I want a 3D printer that can produce e pint of good beer, or even the breakfast toast from a picture, then I might be interested.