Can't use desktop as iPhone-controlled speaker

I am trying to use my iMac (running Mojave) as a speaker for my iPhone 6+ (running iOS 12.4). I have connected my Lightning cable to my iMac, and enabled the iPhone in Audio Midi setup. (All configuration options are greyed out, which seems to be right from what I see on the Internet.) The iPhone reads that it is playing through the dock connector. In the "input" tab of the sound control panel, I see a varying input level. But I hear no sound (sound works for other things on Mac).

Here is what I have tried:

Killing all audio-related processes

Restarting

Connecting phone directly to computer, through hub, or through keyboard

Using internal speakers or headphones for output

When the phone is connected, I can bring up the device in iTunes and play the music that way. But all of the sorting and search mechanisms are greyed out, so it's a pain to try to find songs.