A global "command" to turn text on for accessibility reasons

G'day all

I am new to Storyline and maybe it is too early in my learning curve for this question.  I have been asked by an SME (Academic in the university space) to add a button at the start of a storyline module to allow learners to turn on text bubbles on every slide that otherwise is an audio version of the dialogue in a scenario.  The default position is only the story in audio but if the learner learns better reading and prefers text or is hearing impaired they can choose this option.  With the question of accessibility being more and more to the forefront in curriculum design there are probably tutorials somewhere ... but I have missed them.  I hope it is reasonably straightforward or you will lose me at the moment.  I don't want a script in the form of a document accessed from a button on the player but the text to appear on each slide as it is viewed... providing they have asked for this feature at the start of the module

Please forgive me in advance if this has been covered and I have missed the thread

Regards

Allan