How to reproduce:
1. Download attached project “AudioTest_20183f2.zip" and open in Unity
2. Open “SampleScene” scene
3. Make sure that "Mute Other Audio Sources" setting is unchecked
4. Build the project for iOS
5. Put some background Audio playing (in example Soundcloud playing from Safari)
6. Start the Built project on the Device

Expected results: Background and Built project Audio plays together
Actual results: Background Audio becomes Muted after the Game is opened

Reproducible with: 2019.3.0a1, 2019.3.0b6, 2020.1.0a8

Not reproducible with: 2018.4.11f1, 2019.1.14f1, 2019.2.8f1

Regresion found in: 2019.3.0a1

Notes:
- This issue does not appear on Android - Mono, and IL2CPP
- The issue seems to be caused by the AVAudioSession not being set to the correct category (or any category) in the generated XCode project. Manually adding this, setting the AVAudioSession to the AVAudioSessionCategoryAmbient category in UnityAppController.mm
by adding [[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryAmbient error: nil]; in
- (BOOL)application:(UIApplication)application didFinishLaunchingWithOptions:(NSDictionary)launchOptions