RHVoice is described as 'Speech synthesis method RHVoice uses statistical parametric synthesis. It relies on existing open source speech technologies (mainly HTS and related software)'. There are three alternatives to RHVoice for Linux, Windows and Mac. The best alternative is eSpeak, which is both free and Open Source. Other great apps like RHVoice are Acapela TTS (Paid) and Fala (Free, Open Source).
Fala - A simple text reader is an open source application. A simple software that speaks a text. You can type the text or appoint a file. Fala is just a frontend to festival. Its designed for GNOME, but if you have gtk, pyhton and festival you are able to run it.