Why Is a Violin’s Soundpost Held Without Glue?

Look through a violin’s f-hole and you can see a slender spruce dowel standing beneath the treble side of the bridge. It connects the relatively flexible top plate to the stiffer back. The post supports part of the load transmitted through the bridge and couples the motion of the two plates. It is therefore more than a prop: by restricting motion on the treble side, it helps the bridge rock and directs more movement towards the bass-bar side of the top.

The soundpost is not glued. It is held in place mainly by the pressure created when the strings are at tension. That is deliberate, because the instrument needs to remain adjustable. A violin maker can move the post through an f-hole with a specialised tool. A very small change in position or fit may alter the attack, the balance between strings and the response of particular frequency regions. Glue would lock this relationship in place, leaving little room to respond to seasonal movement of the wood or to rebalance an instrument.

This does not mean that a tighter post simply makes a louder violin. Too much pressure can restrict the top; too little can give poor contact, while the fit of both ends also matters. If the post falls, the violin should not be played at normal string tension because the top has lost important support. The soundpost is best heard as an invisible mechanical joint: it makes no separate note, yet it helps determine how energy travels from bow and string through the bridge, plates and enclosed air before emerging as response and tone.

https://www.phys.unsw.edu.au/jw/violintro.html
https://www.thestrad.com/video/violin-maker-guy-rabut-on-when-to-adjust-your-soundpost/5877.article