Reproduction steps:
1. Open customer's attached project in "unity-audio-tests.zip"
2. Open AudioTest Scene and build for iOS
3. Deploy the application using Xcode
4. Open Memory Tab (spray can icon -> Memory, check images)
5. Inspect memory use
6. In the application press the Load Audio button (MP3, FMOD.sound)
7. Inspect memory use and press the Unload Audio button
8. Switch FMOD.sound to UnityWebRequest in the application
9. Load Audio and inspect the memory use

Expected result: the increase in memory will be the same as when using FMOD.sound
Actual result: the increase in memory will be double compared to using FMOD.sound (read notes below)

Reproducible with: 2019.4.13f1, 2020.1.11f1, 2020.2.0b10, 2021.1.0a3
Could not test with 2018.4.29f1 because of errors in Unity (multiple assemblies with the same name)

Devices tested:
Reproducible with:
VLNQA00310 iPad Pro 12.9 1st gen (iOS 13.4.1)
VLNQA00204 iPhone 5C (iOS 10.3.3)

I could not test this in Editor because the memory usage would not change

Notes:
1. UnityWebRequest also creates an FMOD.sound but after that, the memory allocated by web request is not deallocated
2. Audio Streaming is set to false
3. Some memory is not being deallocated with the Unload Audio button (it is slowly increasing with each allocation and deallocation)