[bug][safari] wasm4.js crash with iOS < 14.1 
### Wasm4 does not work on iOS14.0 or lower

```
Unhandled Promise Rejection: ReferenceError: Can't find variable: AudioContext
```

Tested on simulator with iphone 11Max and iOS14.0 .
It seems that before 14.1 AudioContext was behind a vendor prefix.

source: https://developer.mozilla.org/en-US/docs/Web/API/AudioContext#browser_compatibility
