When presented with the transcript (either while creating a new Campaign or editing an existing one), highlight a section of the transcript to make a pop-up appear

And then give your audio variable a name:

Tip: Use entire phrases rather than individual words in a script

E.g. in the example image above, instead of just using "Samantha", it's better to use "Hey Samantha"

As this will sound better when splicing in different names, e.g. "Hey John", "Hey Amanda", etc.