Way to send Computer audio wirelessly (wifi or bluetooth) to iOS device to be heard on its speakers?
You can buy a bluetooth to audio jack adapter:

https://www.amazon.com/Wireless-Bluetooth-Stereo-Adapter-Transmitter/dp/B00RHKYX7W

This way you should be able to use it as an amplifier using Garage Band amp on your iPhone.

It should work just like so:

https://www.youtube.com/watch?v=RkL0bIx-ESk