When trying to play sound on an iPhone 11 (and also iPhone XR) running iOS 13.3.1 sounds will not play even after unlocking when the phone is in silent mode. There does not seem to be any way to unlock the audio engine when this is the case.
However, if you first play a video texture with audio in response to an event, it unlocks audio and you hear sound from that point on.
See this example.
When in silent mode, no audio plays when clicking the cube on the left even after the unlock audio icon. However, if you click the cube on the right, audio is unlocked and either cube plays audio from then on.
Any (non-hacky) way we could tap into this unlocking to unlock audio even when the iPhone is silenced?