Buttons with Text and Audio: how can I stop all audios from playing at the same time?

You can insert audio into a Marker:

When you do that, the slide will play the audio when a marker is clicked. It will stop that audio if another marker is clicked, so there's no overlapping audio. 

However, that doesn't prevent someone from clicking another marker before the audio finishes on the selected marker. 

One way to do that is to put each audio file on its own layer. Set the Slide Layer Properties to prevent the user from clicking other layers (which includes the base). Also set them to hide the layer when the timeline ends. 

Then add a trigger to each marker so it shows the appropriate layer when clicked.