Buttons are not triggering the audio file

Dear Maria, 

Thank you for your help and I am really sorry to bother you again. In this presentation, when I click the right arrow, the presentation advances perfectly fine.  But after you go beyond the 14th slide, and reach to slide 15, and use the left arrow, it works on the 15th slide. The 14th slide does not show the left arrow any longer, and you cannot return to the previous slide because the missing left arrow. Presentation is provided below. 

  https://360.articulate.com/review/content/df770efb-990f-41ee-b683-f7d3827824f3/review

You do not need to see the entire project but if you do not mind clicking to the right arrow and navigate to the page 15 and then click on the left arrow, you will see the problem.

Any help would greatly appreciated. 

-Mofid