my sonos outsoor speakes will turn down volume after only a few miutes of playing

Sonos doesn’t make outdoor spears, although the do produce an Amp. My suspicion is there is a fault in the wiring of sooner type, where the Amp is getting into protection mode, so it doesn’t overload. That, or the Amp isn’t getting enough air circulation, and is over heating, and consequently shutting down. 

Perhaps a neighbour knows your WiFi code and has the Sonos app.  ;-)

Next time it happens, check the LED on the Amp.  If it is flashing orange then white, the Amp is in fault mode.  This is caused by overheating, a wiring short, or trying to drive speakers with too low an impedance (anything under 4 Ohms total per channel).