Okay here it is :
Just like ome6a1717 said, you need to use a variable that is compared to your text length, in my example the variable is Textlen, for the text I use a third-part plugin for typewriting, but it should be almost the same for C3 typewriting : while the text is animating, it checks if the variable value is smaller than your text length with len(yourtext.text), which will update the variable and play a sound;
The sub-event is for checking if last character is a space or not, by comparing the first character starting from the right of the text; it will play a sound only if the character isn't a space, if you don't need that you can remove the sub-event