I store my music on a NAS (Network Attached Storage) drive. You could also store the files on a PC or Mac. Some routers have a USB interface. And, there are schemes using micro computers, such as a Raspberry Pi.

If your SONOS speakers support Bluetooth or AirPlay 2, you could send directly from a phone or tablet.

I prefer not storing on a PC or Mac because I often reboot my computer and the music would stop if stored on the computer. I’m not a fan of Bluetooth or AirPlay because there are many applications competing for the connection.

Everyone has their own favorite and you’ll get replies from other users.

Here’s what you should know regarding sharing music to Sonos that is stored on another device (PC, Memory Card, NAS) *

First you have to setup a “Folder” in the Sonos app under Music Library so that the music on the external device can be found. I opted to use the IP Address for my NAS that contains my music (See picture below).

There are other options to create the path as show in this link.

After the path to the Folder with your music is created you’ll find it on the Sonos App home page under Your Sources labeled  as Music Library as shown below:

The caveat to accessing music via Sonos on an external device is that the device must be on and unlocked at all times or at least when you want to access it. My NAS is attached to my router and it as well as my NAS remain on 24/7.

 * Memory Cards are typically attached to a PC or NAS via external USB-C or USB card reader.

A NAS is great but expensive, a $15 Raspberry Pi Zero 2 W and a few extra bits for power and storage and running Open Media Vault is a cheap option and easy to set up. A larger, more capable Pi is an option if you want to do more with it, but not needed for Sonos use.

https://www.raspberrypi.com/products/raspberry-pi-zero-2-w/

https://www.openmediavault.org/

As ​@Stanley_4 mentioned a NAS can be more expensive than a Raspberry Pi. However, if you already have a computer that you can leave on 24/7 without going to sleep; or just boot up when needed, the solutions below eliminate any additional setup as might be required with a Raspberry Pi:

USB-C Card Reader

• link

USB-A Card Reader

• link

​@Stanley_4 

BTW...I don’t like Raspberry Pi...I prefer Apple Pie...including my cell phone, tablets and computers

There are alternatives, Orange Pi is one I've seen.

https://www.zdnet.com/article/best-raspberry-pi-alternative/

https://www.xda-developers.com/devices-you-can-buy-instead-of-a-raspberry-pi-5/

None have the level of new user support the Raspberry Pi offers, it is intended to be an educational computer for younger kids, most others are aimed at higher skilled folks.

The Radxa and Zimaboard look particularly attractive if your wallet and skills are ready for them.

For round pie my favorite is rhubarb, the greener the better, and I'll stick a fork in you if you add strawberries! Add a scoop of vanilla ice-cream, one where you can pronounce all the ingredients, and I'm happy.

Upload the files to iBroadcast.com and add that service to your Sonos system.