Why do you use a serial for sending phonemes?

Looks like really unixish to me.

Originally, the Amiga was capable of doing speech synthesis.

Some hint of how things were done in the past

http://wiki.amigaos.net/wiki/Narrator_Device

and

http://wiki.amigaos.net/wiki/Translator_Library (but totally OS4 way of doing thing, read not the original AmigaOS way).

Hope this will give you some ideas.

AmigaOS is not like Unix.

A good starting point : http://library.morph.zone/Main_Page

Also, provide some nice example to write clean GUI with MUI.