It is not clear what the plugin does.

espeak-ng outputs audio encoded as audio/wav.

Does the plugin pipe default audio output from espeak and/or espeak-ng through a native application to output a different, user-selected audio codec?

If that is the case an intermediary application is not necessary, for example, to convert default audio/wav output stream to audio/ogg;codecs=opus (audio/x-opus+ogg)

$ espeak-ng --stdout 'test' | opusenc - -