"auto","autodetect"
"al","Alva"
"an","Android (text to speech engine)"
"bl","BrailleLite"
"cb","CombiBraille"
"es","eSpeak (text to speech engine)"
"fl","FestivalLite (text to speech engine)"
"fv","Festival (text to speech engine)"
"gs","GenericSay (pipes to /usr/local/bin/say)"
"mp","Mikropuhe (text to speech engine)"
"sd","SpeechDispatcher (text to speech server)"
"sw","Swift (text to speech engine)"
"th","Theta (text to speech engine)"
"vv","ViaVoice (text to speech engine)"
"xs","ExternalSpeech (executes /usr/local/bin/externalspeech)"
