Can I send a midi message from MATLAB to an external source audio plug in? For instance a VST plug in like Serum? There does not seem to be any interface for doing that in the audioPluginSource class interface.
If you want to use MIDI (as opposed to loading a plugin), have a look at midimsg. Just note that we don't support loading VSTi plugins in MATLAB at this time, but it looks like that is not what you are trying to do.