TTS on button click

I am a bit confused, will guess a little bit.

If you use TTS you create an audio clip (wav format). That audio clip will indeed automatically be inserted as slide audio (visible on the timeline), but also appears in the Project Library.

If you want to play that clip in another way (with 'Play Audio' or as Object audio), follow this step-by-step workflow:

• Use the right-click menu on the Audio timeline (Timeline panel) to  'Find in Libarry'. You may need to edit the name of the audio clip, so that it is easier to identify. Or you have to write down the generic name, up to you.
• Delete the audio timeline in the Timeline panel
• Create the next text needed in that slide with TTS and repeat the workflow.

When you have finished with all the audio for the slide, you'll have identified all audio clips in the Library.

Now you can set up the interaction. I don't know how you change the text in the text container  (I avoid the word 'box' because a Text Entry Box exists). If it by using a multistate object, where you show the correct text by switching to the correct state, you can add the audio clip directly to the state. If you use a show/hide workflow for the text, you can attach the audio clip as object audio to the text. If you use still another workflow, let me know and I'll try to give you a solution.