I don't have an answer to your specific question (and I'm curious if there is a way to do that), but I do think there's an alternative way to solve your problem. Get a cheap bluetooth speaker that will work while plugged in to mains (some won't play while the battery is charging). When you're listening to music, you can hear Alexa through your stereo system. When you're done listening to music, say, "Alexa, connect," so that you can hear Alexa through the bluetooth speaker even when your stereo is off. When you want to listen to your stereo, say, "Alexa, disconnect."