TTS Service is described as 'This is a text-to-speech library intended for use by other applications'. There are five alternatives to TTS Service for a variety of platforms, including Windows, Linux, Online / Web-based, iPhone and iPad. The best alternative is Read Aloud, which is both free and Open Source. Other great apps like TTS Service are eSpeak (Free, Open Source), ReadSpeaker (Paid), Voice Dream Reader (Paid) and NeoSpeech (Paid).
Voice Dream Reader lets you listen to any document and ebook using text-to-speech. Text-to-Speech - One premium Acapela voice of your choice free with the purchase of the app. - 36 built-in iOS voices in 27 languages are also available for free.