App Transport Security issue with AVAudioPlayer loading local file objective-C, XCode 9

I have an app which loads a bundled m4a audio file as a local resource and it has worked well for many years now. I'm updating the app to iOS 11.3/XCode 9.3 and it is now failing on iPad (works on iPhone) when I press my play button:

2018-05-13 20:45:24.437626-0700 my app[6175:218735] App Transport Security has blocked a cleartext HTTP (http://) resource load since it is insecure. Temporary exceptions can be configured via your app's Info.plist file. 
2018-05-13 20:45:24.437791-0700 my app[6175:218735] Cannot start load of Task <117E064E-ABB3-45F2-8D64-76397B140092>.<0> since it does not conform to ATS policy
2018-05-13 20:45:24.437948-0700 my app[6175:218732] NSURLConnection finished with error - code -1022

My code:

  
 
NSURL *medURL   = [[NSBundle mainBundle] URLForResource: @"MyFile" withExtension: @"m4a"]; 
NSError *playerError = nil; 
AVAudioPlayer *newPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL: medURL error: &playerError]; 
self.appSoundPlayer = newPlayer;  

// "Preparing to play" attaches to the audio hardware and ensures that playback 
//      starts quickly when the user taps Play 
[appSoundPlayer prepareToPlay];

It fails on the

prepareToPlay
.

Reading other answers I've found a lot of information on the ATS error so I added this to my plist file for the app, cleaned, rebuilt and ran it -- but I am still getting the error:

  
 
NSAppTransportSecurity 
 
NSAllowsLocalNetworking 
 
NSAllowsArbitraryLoadsForMedia 
 
NSAllowsArbitraryLoads 
 
NSAllowsArbitraryLoadsInWebContent 
 
NSExceptionAllowsInsecureHTTPLoads

I can't use the NSExceptionDomain key because this is a local resource, there is no domain. How can I fix this?

Note: to be clear, this file is bundled into the app and has not been downloaded at any time. This is not an "http" url, it is a "file:" url.