The audio devices in Tobii Communicator are all set to "Windows Default Device" except "Skype and Bluetooth Mobile Phone Calls" which is set to "Windows Default Communication Device". Pressing the play button next to each device will play a small signal. If you have a Bluetooth speaker connected, it might add a secondary audio device which might be set to Windows Default Communication device. In this case, and after playing the "Skype and Bluetooth..." sound, the others (Windows Default Device) will just produce a quiet scraping noise. After waiting some time, approximately a minute, it starts working again.
Solution (here as an example for a Boombotix Rex speaker):*