Cannot use iOS device as sound input
Solution 1:
You need first something to capture the input, which can then be routed to an output.
Launch Quicktime Player, elect New Audio Recording & next to the red dot, select your input.
Turn up the volume slider underneath.
This should then route to your outputs, allowing playback. You don't even need to start the recording.
There will be latency, maybe as much as half a second.