Quiz features - help with basic questions

If this is the way you want your quiz to work, just place your video clips on the slides BEFORE each relevant quiz slide. Insert the video as Slide Video so that the slide will be extended to the length of the video.  It will then automatically progress to the next slide (the quiz slide) where the learner can answer the question.  I would still suggest that you have a text version of the question on that slide as well though.

If you put a video on a quiz slide it will ONLY be able to play behind the answer captions because they are always on layers above anything else on the slide.  However, your issue is more likely to be getting the videos to play as feedback.  You could use the ON Success and On Last Attempt events to SHOW a given video clip, but in this case it could only be inserted as Event Video, not Slide Video.  To get it to play as soon as the user submits their answer, you would need to disable the normal feedback captions.

Personally I'm not a fan of the approach you're talking about here.  Using so much video is likely to bog down your project and make it run very slowly for end users that may not have great bandwidth.  Only use video if there is NO OTHER WAY to effectively communicate the same information.  If your video is just for eye-candy sake, give it up.