Paste or type any text and press Read aloud. The tool uses the speech synthesis built into your browser (the Web Speech API), so the voices are the ones your device and browser provide: the same text can sound quite different on an iPhone, a Windows laptop and an Android phone. You choose the language and the voice, set the speed, pitch and volume, pause and resume, and, where the voice reports its position, see the word being spoken highlighted. Long text is read sentence by sentence so the engine does not stall halfway.
Why the voices differ from device to device
The Web Speech API does not ship voices; it asks the operating system and the browser which ones they have. macOS and iOS offer many voices, some only after you download them in the system settings. Windows has a few built in and more as optional speech packs. Chrome on a computer adds a set of Google voices that work through Google's servers; Edge adds Microsoft's online voices. Android uses whatever text-to-speech engine is installed. If you see no Vietnamese voice, install one in your system's language or speech settings and reload the page. More about this, with what to check on each system, is in our article on how text-to-speech voices differ by device.
Browsers play speech synthesis straight to the speakers and do not give the audio to the web page, so no website using this API can save it as an MP3 honestly. Sites that offer a download send your text to their own server and synthesise it there. If you need a file, you can record the computer's sound with a separate app, or record a voice-over yourself with the voice recorder and tidy it up with the audio trimmer.