You are running this under the special krzmbrzl plugin mumble branch?
When you start mumble from the terminal, output should occur: can you please send that to me?
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Edit: the mumble chat box will display clearly if the plugin could be loaded or not. If neither message shows up, you are not using the needed mumble version/branch (assuming the fgcom-mumble plugin is really in the plugins folder)
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Edit: to clarify: you need to compile mumble from the plugin branch.
The ordinary mumble 1.4.0 master branch does not yet contain the new plugin framework and thus cannot load the fgcom-mumble.so!
The plugins you see there are just positional-audio plugins which is a different thing.
Code urls etc in the compile instructions: fgcom-mumble wiki