Forgetting and re-pairing worked for me.
I have an iPhone 6 going to a Vizio 5.1 soundbar. The sound is better when using my Apple TV, but doing so ruins gapless playback with a 3-second pause between tracks. I listen to live music where this really kills the vibe, so the Bluetooth mode is preferable. Suddenly the volume was just nonexistent, even though it was still plenty loud going through Apple TV and the Optical cable input from the TV. I followed the above suggestion, went into my iPhone's Bluetooth settings and "Forgot" the Vizio. Then I did a re-pair with both devices, and the volume is back to full.