Assignment Isn't showing as completed. Could Next buttons being reviewed be the problem?

If a course resumes at a point earlier than where you exited it, that could be caused by using a publishing standard that doesn't allow enough room to store the suspend/resume data. SCORM 1.2 has a very low limit for that data.

• If that's the case, the fix is to publish with a standard that has a higher limit. Check with your LMS folks to see if SCORM 2004, edition 3 (or later) is okay.
• Here's more info on this: Storyline: Exceeding SCORM Suspend Data Limits - Articulate Support 

You can also try to lower the amount of data needed to resume. For example, that can be done by using fewer variables and by not saving states. But it still may not be possible, and those sort of changes can have a negative impact on how the course works.

FYI: you don't need any triggers or variables to require a user to view every slide in order. Just set the Player's Menu Navigation to Restricted. The program will automatically disable the Next button the first time a user visits the slide, and it will enable the Next button when the timeline ends. This works even if you don't show the built-in Menu. 

• Here's more info about that and other ways to control the Next button: TIP: Controlling the NEXT Button 101 - Articulate Storyline Discussions - E-Learning Heroes 

Also, you don't need a trigger to delay the start of the audio. Just drag the audio to the desired start time in the Timeline: