Yes, ST is Smartthings. Since this is a Smartthings forum, I assumed you had a hub. To use your Sonos directly from Alexa create a routine as follows.
When Ring motion or press,
Add action - Use custom action and just type in a command to Alexa. "Alexa play doorbell on Sonos speaker. " It has to be the name you gave your speaker in Alexa when you added the device.
Basically the same command you would say to Alexa to play whatever you like on your speakers.
It will ask you to choose a device to play it from. Pick any device. It doesn’t matter since the Alexa command should override this choice.
I tested it and it worked for me, so give it a try.