Let's jump straight to the most likely problem...
The iPhone has a mute button that you might have toggled to turn the ringer off on your phone, and only do silent/vibrate notifications for calls. When you have your phone ringer turned off, asensei will also mute any coaching or audio from the app.
So first thing to check; toggle the mute button on the side of your iPhone.
If you're still not hearing music, chances are you've inadvertently paired with another audio device.
- Go to Settings, Bluetooth and check you're not paired with another audio device
If you're still having issues, reach out to as at firstname.lastname@example.org