To stream audio only in iOS 11 you'll need to do the following.
1. Make sure Reflector 3 is up and running.
2. Open up your iOS device's Control Center by swiping up from the bottom of your screen.
3. Tap the two lines in the top right corner of the music player in the Control Center.
4. Select the device you wish to send audio to in the list of generated devices that appears after tapping the icon in the music player from the previous step.
5. Press the play button in the device menu/music player.
6. At this point, your device should be sending audio only to your selected destination.