I have no doubt that the best glue for instrument-making is hide glue.  It resists high temperatures, vibrations are transmitted effectively, It doesn’t dull edge tools, restorations can be effected more easily on instruments that were made with it and there are many other advantages.  In my observations, if similar instruments are made with different types of glue, the hide glue version will sound better.  The joints on a musical instrument are not glued with high pressure but at the same time it is important that the glue line be as thin as posiible.  Several factors can help us achieve this: thinning the glue appropriately, making sure the temperature of the glue does not drop too quickly so it can still flow and making sure that the excess glue has somewhere to go.  For determining the right viscosity of the glue it is a matter of experimenting with more or less water in the glue.  Heat can be maintained either by heating the pieces to be joined or in the case of thin stock, heating the joint once it is closed.  Giving the glue somewhere to go is especially important with hardwoods which might absorb less of the glue and cause a thicker layer which can separate due to its low cohesive strength.  This is done in a number of ways:  you can rub the two pieces together which will force the glue out of the joint (this doesn’t work on something like a fingerboard as full contact is not made until clamped). Hiigh, even pressure helps but you can still trap pockets of glue in the middle of a large piece.  Making a “key” on one of the pieces is useful but not for the reason that many think.  Using a toothing plane or a knife to make grooves on a piece give the excess glue somewhere to collect.  Some emulate this technique thinking that the grooves let the glue grab the wood but that is not how glue works.  Of course if you do this you have to leave plenty of wood in contact along the glue line, if you rough the surface with a heavy grit sandpaper for instance you are NOT improving adhesion.