Hi jvans,
You may need to select the Bluetooth speaker within the Music app to have audio play through the speaker. Here are instructions that might help:
"Play audio from iPhone on a Bluetooth audio device
- On your iPhone, open an audio app, such as Music, then choose an item to play.
- Tap , then choose your Bluetooth device.
- While audio is playing, you can change the playback destination on the Lock Screen or in Control Center.
The playback destination returns to iPhone if you move the device out of Bluetooth range.
For information about protecting your hearing from loud volume while listening to headphones with iPhone, see Use headphone audio level features on iPhone.
WARNING: For important information about avoiding hearing loss and avoiding distractions that could lead to dangerous situations, see Important safety information for iPhone."
Set up and use Bluetooth accessories on iPhone - Apple Support
Cheers!