Where did you; as you say, register the Arc with Alexa?

I registered it with my Amazon account.

I have a recording of the response I get back from Alexa. However this does not allow media attachments.

Try the following if you haven’t done so….

In the Sonos app  > Settings > System > Room/Zone with Arc > Voice > Amazon Alexa > make sure Alexa is enabled.

Let us know what happens.

I don't see the option Room/Zone with Arc when I go to System settings in the Sonos App.

Room/Zone isn’t an option. LOL.

Just look for your Arc. It’s the room or zone where the Arc is located. (i.e. Living, den etc). 

Let us know what happens

I see the ARC in the living room. It has Amazon Alexa set up as voice control. When I go into the Alexa App I have enabled both Pandora and SiriusXM. I set this up with my Sonos Move in my other home back in January and I had no problems. Everytime I ask Alexa to play Pandora it responds “To do that first enable the skill in the Alexa App, then ask me to discover devices”