System - Pi3+. Fixed IP over wired Ethernet. Ver 2.692. On my system if I disable WiFi get no Webradio or "Volumio Selection" on Web Radio browse page. No station list populated. If trying to play a previous station get a message "Failure to decode stream........ CURL....." It appears that Volumio can not reach the streams unless wireless connection is enabled and connected. This is repeatable. Also tried on a fresh flash image.
Fixed by setting the gateway to match my router in the Network Settings/Wired Network set up. By default the gateway for Wired Network was set to 0.0.0.0 whereas Wireless network was set to the correct 192.168.X.X for my router.