Sorry to hear you're having issues with audio playback on Iphone. Would you mind checking this article: https://helpx.adobe.com/in/xd/kb/known-issues.html#mac-only under "97262 - (Safari only) Audio playback action in Prototype mode is not functioning due to console errors" and try the workaround solution mentioned?
Oh sorry about that. Is it possible for you to share the file with me over a private message so that I can investigate at my end? Please upload the file to a shared location such as Creative Cloud, Dropbox and share the URL with me over a PM.