I can't hear the audios / sounds
It is recommended to use an updated version of Chrome browser or Safari, although it might work in other web browsers
If you are using a mobile device or tablet, you might need to install the corresponding language.
This may vary in different devices, but in general you have to go to Settings > Language & input > Text to speech output and install the language.
If you are using a Chromebook, please take into account that this device is not compatible with the voice synthesizer.