I believe that the difference in sound between metal and wooden pipes
is mostly due to the shape.  Sheet metal can be rolled and soldered to
make round pipes, whereas wooden slabs are easily glued in polygonal
shapes.  A simple wood pipe should sound the same if duplicated in metal.

Research comparing different pipe resonator materials was conducted
circa 1959 for a Dutch organ firm.  Everything from cast iron to
redwood to asbestos tubing was tested.  (PVC wasn't invented yet!)
The tests showed that all materials produced the same tone except the
redwood and asbestos, which was very soft and porous.

The Dutch firm then concluded that there was no good reason to
abandon the traditional materials.

Robbie Rhodes