How to reproduce:
1. Open the attached project "case_1259748-iOSAudioRepro.zip"
2. Open Player settings
3. Make sure that Mute Other Audio Sources is enabled and Disable Unity Audio is disabled
4. Play some audio on an iOS device (e.g. Soundcloud in Safari, Spotify..)
5. Build the project
6. Deploy the Xcode project to an iOS device

Expected:
Background audio is muted

Actual:
Background audio is not muted

Reproduced on:
2020.3.8f1 2021.1.7f1

Not reproduced on:
2019.4.26f1 2020.3.7f1

Notes:
-On 2020.3.7f1, background audio is muted always if Mute Other Audio Sources is disabled or enabled
-Tested on macOS Big Sur
-iPhone XR iOS 13.2.3