What is Sinsy?
Sinsy generate audio files out of lyrics annotated MusicXML scores. A web service demo is available online, featuring a more recent version than the one found in the official public repository.
Comments and Reviews
Said about Sinsy as an alternative
Voice synthesis in Sinsy is just as easy as in Vocaloid, despite having to create MusicXML file with MuseScore (which is also FOSS)