https://wiki.archlinux.org/index.php/PulseAudio#Flash_ContentSorry to write this but that didn't fix it - I installed pulseaudio-alsa but flash does still not use the headset.
install and run pavucontrol and see if flash is using pulseaudio (it should show up in the "playback" tab) then maybe move the flash stream to the bluetooth headset device.
-- дамјан