Turn off the speaker.
Turn off Bluetooth on your source device.
Use the Bluetooth menu to disconnect the speaker.
Some Bluetooth devices allow you to shift the audio from the device to the phone and back again, but that depends on what you're working with.
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For most apps, just turning Bluetooth off will shift the output to the speaker. Just pull the Notifications down and tap the Bluetooth icon. (If you have an app that doesn't work with, try keeping it on speaker, but running BTmono to shift its output to Bluetooth - just tap "on". Tapping "off" in BTMono will restore the output to the speaker.