How to prevent audio from playing when returning to slide

I'd love to help you out but I'm not sure how you have the slide setup to make a reliable suggestion. From just the way you described I imagine the solve to be this. 

1. Create a true/false variable with the default set to false, let's just call it "Complete" on the slide with audio.
2. Create a trigger to play the audio on the slide when timeline starts with the condition "if Complete = false"
3. Create a trigger that changes that variable "Complete" from false to true when the media completes.

Here may lie a problem.
- If your user clicks Next before the audio has a chance to play through completely, the trigger in step 3 will not fire and you will encounter the problem you described. If you want to prevent this, you have to make sure that your learner listens to the entire clip. So here's what I would do to force the learner to listen to the audio clip at least once.

Create a trigger on the slide with the audio that disables the Player Next button on the slide if the variable "Complete" is "false". This makes sure that it doesn't fire when you revisit the slide after completely listening to the audio on a previous attempt. 

Create a trigger on the slide to enable the player Next Button when the audio on the slide completes. 

So what should happen is:

• User visits slide for the first time, Next button is disabled.
• Audio plays automatically, has to be listened to completely before next button is enabled,
• User goes to the next slide

User revisits slide with audio, audio won't play, Next button is already enabled.  

As Wendy mentioned above, this will work on a slide-by-slide basis so your "Complete" variable has to be unique for each slide with audio you want to behave this way. i.e. "Complete1", "Complete2" etc... since variables are global.

Again, best guess, if this doesn't fix your issue, upload at least the slide you need help with for us to get a better picture.