Current Git version of ScummVM
The FluidSynth settings button appears on the MIDI tab of both the Options and the Edit Game dialog, but only actually works from the Options dialog. I don't remember if it was ever intended as a per-game setting or not.
As of 40a663d58990e309d7673fa64ab8b194aaac473b we do not display the button in the edit game menu anymore. The change was contributed through pull request #715.