There's a merged pull request on the void-packages repository

mumble: fix plugin path
https://github.com/void-linux/void-packages/pull/27065

Description:
https://github.com/mumble-voip/mumble/blob/1.3.3/src/mumble/Plugins.cpp#L212-L220

Mumble was searching plugins in `/usr/bin/plugins/` because `MumbleApplication::applicationVersionRootPath()`(basically the `QCoreApplication::applicationDirPath()`) returns the path of the application executable.