I think I found the answer. I needed my phone's music to be visible in my Google Play Music app. (I used the Android Music app to play my music, it did not do "OK Google" well.)
In Google Play Music, my playlists were nearly all empty. I sync my iTunes music via iSyncr to my phone's SD card, which my Android Music app saw and my Google Play Music app did not. So in the Google Play Music app...
1. I hit the menu icon (three bars in upper left corner)
2. I hit Settings, Storage location.
4. For Choose storage location, I selected External.
5. I also hit Settings, Refresh. That might have helped.
I can't get it to play my playlists. Yet. Works on my iPad's Siri! But what's with "Hey, Siri" (using Siri without pressing the Home button) only working if my iPad Air is plugged into USB power? Grr! Another reason to buy an iPad Pro. I'd better hurry up and finish my novel (thus becoming the next Stan Lee).
P.S. My phone is still fussy. It plays most songs I "OK Google" request, like Mona List Overdrive (from The Matrix), but it would not play Super Chicken (theme song of a great oldie cartoon). And both those songs are on my phone. I then tried, "OK Google, play Super Chicken from my phone." It did not find the song. Why? Because on my phone, the title is one word: "Superchicken."