The button (or end pin in the case of celli and bass’) may seem an insignificant part of the violin. A part that merely holds the tail piece in place. This is mostly true but there is one aspect that needs to be secure; how well it fits and seats in the reamed hole will affect the transfer of sound throughout the instrument.
There are, however, small modifications to some modern buttons and endpins sold today that allow you or the luthier to be able to see the sound post while it is in because it is hollow.
As a rule they are made up of the different standard woods used for fittings although some prefer composite materials that are believed to increase or enhance the sonic properties of the instrument. Here we have no known proof that we feel comfortable quoting to substantiate such a claim.
Note. The most principally important thing for ALL instruments is perfectly flush contact. Same as electricity, sound works with the same principals.