No, Sonos itself doesn’t store music. It plays music that is streamed to it. Sonos just doesn’t have the memory installed on the speakers to store music. 

You could potentially store the music on whichever mobile device you use, and use “on this moblie device” (depending on DRM factors), Bluetooth (depending on which Sonos speakers you use) or AirPlay 2 (depending on which phone you use) to play to the Sonos. 

I have an attached NAS (a little USB stick connected to my router) that holds my old MP3 collection. I feel like Sonos could figure a way to detect this drive and make its own cache folder on it to store offline pandora music, as if it were my mobile phone. 

Those downloads are copy protected, and most services refuse to license their copy protection to 3rd parties.  However, Pandora allows you to play to Sonos directly from the Pandora app, which should enable you to play those local files.  

I figure they get cached in an encrypted format, just like how they get saved on your mobile phone. Seems like that is not a blocker. 

In order to play them from the 3rd party Sonos app, Pandora would have to give Sonos the decryption algorithm, and they aren’t going to do that.  As I said, try playing them to Sonos directly from the Pandora app. 

I doubt the casting  from Pandora would actually work for locally stored files, but have not tried it to confirm.  

Huh?  The whole reason it’s encrypted is to block you from playing the music outside of the Pandora app. Do you have an iPhone, have you tried using airplay?

It would be played through the Pandora sub-app that already exists inside of the Sonos app... 

There is no “Pandora sub-app”.  It’s a Sonos app. Pandora contributes some API work, but in order to play encrypted files, Pandora would have to give the decryption algorithm to Sonos, which isn’t happening. 

The “Sonos sub-app” you speak of os merely a pointer of the standard, every day Sonos player to the server that Pandora stood up, and applied the Sonos API so that their data could be sent to the requests sent by Sonos controllers. 

There is no difference of the “player” part of the software between any of the companies that work on your Sonos devices. The only “difference” is the pointer in the Sonos software to whichever server the streaming company has set up to respond to requests via the Sonos API. 

You may want to read through some of he Sonos partners page.

While it might be “cool” if each company had it’s own separate player in Sonos, the amount of memory required to do that would be prohibitive (in the US, I’m counting around 100 services).  Imagine, how many “players” can you fit on your phone? The Sonos devices likely has much less memory than your mobile and and computer devices do. THey’d be pretty expensive to have enough RAM to store all of the individual players for each streaming company, then add in the storage for the music you appear to want it to be able to store, too.