Hide quiz submit button until audio finishes duration.

Some details are lacking. The most important: are you talking about a responsive or a non-responsive project? I have described a tweak in a blog post for both situations:

https://elearning.adobe.com/2019/04/quiz-tweak-2-slide-audio/

It is much easier for a non-responsive project, because of the possibility to stack objects. For the Fluid Boxes it is a lot tougher to achieve, and needs extensive testing. I do double-check every workflow posted on my blog consciously. 

Since you are using random questions, this needs to be done for each question in the question pools. The possibilities and limtations of random questions are described in this blog:

http://blog.lilybiri.com/random-questions-dos-and-donts

Do not forget to move the pausing point in each question near the end of the slide, or to change the default Continue action to 'Go to Next Slide'. That pausing point is not stopping slide audio, I presume you are talking about slide audio because that is not mentioned in the question.

I had a client a few years ago who wanted a similar situation. The only solution I could come up with was to position the submit button outside of the slide. I set the motion path start time to an amount past the audio narration. I also set the duration of the motion path to be so short it essentially just appeared. It worked to my client's satisfaction but in the end, we returned to a normal question slide because employees complained. I don't recall if they were in a pool and displayed through random question slides or not but you can certainly try that approach.