Install passive outdoor speakers outside and power them with a Sonos Amp. Then use the Alexa app on your phone or a Sonos Roam outside to control the speakers using Alexa.

Along the same lines you can place one or more echo dots outside for control rather than your phone.  I’ve found that echo dots do fairly well outdoors,  if you place them under eaves and generally try and protect them from the elements.  I have two in my backyard right now, probably add a third in the next few weeks for better coverage. I have had a couple dots fail in the past, but their pretty cheap to pick up new ones.