User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:55.0) Gecko/20100101 Firefox/55.0 Build ID: 20170512030203 Steps to reproduce: Set audio source to Logitech G 430 Headset (has its own usb audio controller), start FF Nightly and open youtube. Start a video and switch back to playback settings on windows. Change audio source (sink) to another device: for instance to NVIDIA HDMI Audio which is connected to a monitor with speakers connected to it. FF will still play on the Logitech headset. Actual results: FF did not follow to the new audio sink.