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

Notes:
- This issue does not appear on Android - Mono, and IL2CPP
- This issue appears only on iOS
- Could not check on Unity 2018.3.0a11 because Built project crashes on launch
- Workaround: If the AVAudioSession category is manually set to for example AVAudioSessionCategoryAmbient in native iOS code (tested with [[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryAmbient error: nil]; ), the background audio will continue playing after the app is launched

Tested with:
- iPhone 7 Plus, iOS: 12.0 - Reproduced
- iPhone X, iOS: 11.3.1 - Reproduced

Reproduced on 2018.3.0b1, 2018.3.3f1, 2019.1.0a14 and 2019.2.0a1
Not reproduced on Unity 2018.2.20f1 and 2018.3.0a10
Regression on Unity 2018.3.0b1