well, confirmed I can fix it by turning the speaker off, re-plugging the dongle and then running:
net stop hfgservice net start hfgservice net stop audiosrv net stop audioendpointbuilder net start audioendpointbuilder net start audiosrv
then turning the speaker back on. needless to say, this is a bit tedious (though having the BT dongle in a front USB port within arm's reach helps), and I ought to finally get a more expensive dongle so this isn't necessary.