This is not working cause murmur-static_x86-1.4.230.tar.bz2 has not been published yet https://dl.mumble.info/stable/
(And seems like static version is dropped mumble-voip/mumble#5453 (comment))

Just check your PR by run make in stable directory

We are already in the process of establishing a official Docker infrastructure.
See https://github.com/mumble-voip/mumble-docker if you don't want to make the same work as me.
Maybe you can contribute ?