Hi Ian
The first thing you need to know is that the Brennan B2 does not use music tags. It does not mind them being in the music files and if they are there then using the B2's NAS, an external system (eg your Sonos) will be able to use them to build an index. So if your music files contain tags this will be fine.
The B2 stores and names its music holdings using a heirarchical directory system with the names of individual directories denoting Artist and Album names. The structure is outlined on the Brennan web site. This means that unless you use a clever naming system or make your own playlist catagories such as " genre" are not available.
So, provided you arrange your music into the required directory structure, loading it onto the B2 is simple, you write it to a FAT32 formatted USB disk (format the empty disk to FAT32 using the B2 to do this first) then plug the disk into the back of the B2 and load the music using the B2's import function.
FLAC compressed music files will be fine.
The B2 will connect to your house router using WiFi, it also has an Ethernet socket internally ( or you can use a USB ethernet adapter). But note to work properly the WiFi strength and quality needs to be good. Many of us prefer to use Ethernet to the B2 rather than WiFi.
You can use the B2 as a NAS and drive your music from the Sonos App
And or
You can push music to your Sonos from the B2. The B2 will "see" all the Sonos speaker names you set up with your Sonos App and can send different music to each of these names at the same time. To get the same music to all the Sonos at once you need to send it to one then use the Sonos app to get this to go to the rest.
Please understand the connection between the B2 and Sonos is between the B2 and the Sonos system, not direct to Sonos speakers themselves.
For instance if you play a radio from the B2 the B2 will ask the Sonos to do this. So if you turn off the B2 without first stopping the radio, it will continue to play from the Sonos system. This means that the Sonos app will show you what the B2 is doing.
Hope this helps.
Fred