I believe the brittle nature of dried animal glue creates a solid, inflebixble joint, whereas modern glues like PVA are somewhat rubbery even when cured and this flexibility seems to bleed off sonic energy and vibrations.

Paul Poletti's website has a Glue section at http://polettipiano.com/Pages/pag1engpaul.html.  Among his remarks about modern glues is this:

"...because PVA glues are always slightly rubbery, even when they are technically "hard", it's like putting a thin piece of sound absorbing rubber between all the pieces of the instrument. Many guitar, violin, and lute makers testify that using Titebond kills the sound of even an otherwise good instrument."

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