Hi Daniel,
I see the point you are making. I don't have a Sonos system, but have had a look at the Sonos set up instructions, both on the Brennan site and on the Sonos site.
As far as I can see, the Sonos system is set up from an app running on PC/ Mac/ Android/ iOS. On a PC, this is where NAS/ SMB1.0 is required to be running, but only to get the NAS path loaded into the Sonos system. Once the Sonos app (on the PC) has loaded the network access path into the Sonos system, then the PC will no longer be required for Sonos to operate because the Sonos and Brennan will communicate directly with each other, using NAS.
During the Sonos setup, the Sonos PC app asks for the music folder network path which will be '\\BrennanB2\Music'. The Sonos app (on the PC) will send this to the Sonos system which will enable the Sonos system to function directly with the Brennan. The '\\BrennanB2' is a global network reference across your home network, and can be accessed from anywhere on your local home network.
Without SMB1.0 running, the PC can only 'see' the Brennan music folder as a mapped drive i.e. 'Z;\Music' which would not work for the Sonos system as it will not recognise this path because the 'Z:' drive is only a local reference on the PC and will not be visable on your local home network outside of the PC.
Please note that I am theorising to some degree, as I don't know the Sonos app. Perhaps someone who has the Sonos system running could comment or correct my note here.
Also, for Mac OSX users, some of the later versions of OSX do not provide a reliable version of SMB1.0 due to some sort of disagreement between Apple and Samba (who provide SMB1.0). As a result, Apple tried to develop their own equivalent of SMB but failed to complete the project. There are some alternative solutions available; if you google 'MAC OSX SMB problems' you will find plenty of material on that topic.
Regards,
Peter.