It knows because the location is stored on the speakers themselves as it is the speakers that perform indexing and store the actual index. 

What version of firmware are your speakers on? Look in about your system. 

If you are not on 16.3.something then you may regret uninstalling once you are on 16.3.

What is the issue you are trying to fix? 

@Ian_S Thanks for the pointers. All speakers are on version 16.3.3 and the iPhone app is v80.09.08

The problem I have is that the Music Library does not appear on the Home page of the app.  It does show on the Windows app on my laptop but the indexing is messed up with every track on a compilation appearing as an album in it’s own right.  

The only way the messed up indexing can be fixed is when Sonos (eventually, it’s been three months!) deliver a speaker firmware fix, as it’s the speakers that do the indexing. Hence I said if you haven’t already upgraded to 16.3 you’ll regret doing so!! 

The only way to completely reset would be to factory reset the speakers as well, and then probably delete the app again and when you set it all up, select the create new system option. 

However, the fact that your speakers see the library and the Windows app does, points to the new app being more of the problem, and a full reset may leave you no further forward. I would be tempted to wait for the next app drop first as I believe the ability to cope with more network configurations is slowly improving. (Not fast enough though!) If that doesn’t work then it might be time to look at the network to see if there is anything that might help, but as Sonos won’t tell anyone what changes they’ve made, it’s a bit of a guessing game trying to figure it out...

If you force close the App the library will probably appear on the home page. Don’t Factory Reset without further consult. Factory Reset will induce lots of extra work and it rarely resolves fundamental issues such as this. I suggest waiting for another update — and wait for the early adopters to report their experience.

Thanks for the suggestion but the app has been closed many times (there is no “force close” on iPhones) and has even been deleted and re-installed.  However, the library still fails to appear on the Home screen.

I’ve also noticed that even though the NAS library is listed as a Shared Folder under “Music Library”, if I click “Detect Music Library” I get a “Music Library was not found” message :

@essenby A new app & firmware has just been released. Have you tried these yet?

@Ian_S - When I “Check for updates” it tells me my system is up to date. 

Speakers are on firmware 16.4 and iPhone app is 80.09.08 - but still no Music Library on my Home Screen.

@Ian_S 
Hi.  Sorry to dredge up an older post again but I wonder if you may be able to help explain something for me please?   I used to host my library on a Buffalo Linkstation but had to move it to a QNAP NAS when Sonos removed support for SMBv1.  Yesterday I removed music library folder from the app, then removed the app from my iPad and installed the recent 80.11.51 update. When I went to add the NAS shared library the name of the old Linkstation appeared fleetingly in the shared folder box.  Could one (or more) of my speakers be remembering this?  And could this be a reson why my library indexes are a bit screwed?  Couls it also be behind the fact that “Your Sources” appears and dissapperas at random on my home screen?

How many Sonos speakers do you have and what are they? 

@Ian_S 
Apologies for the delayed response.  Speaker list as follows:-
1 x Move

2 x Roam (normally on and fixed but are moved around occaisionally)

1 Playbar - optical connection to TV

2 x Play 1 - connected as Left and Right surround for the Playbar

All speakers are on Firmware 16.4.4 (build 81.1-58210)

I also have a Play 5 controlled from a separate S1 app but on the same network.  That is on version 11.14(build 572151190) of the S1 firmware.  It is not connected to the music library but used to be connected to it when the library was on the Linkstation (SMB1 only)

Further info - I did a “Scan for new Content) over the weekend and at the end of the scan, the old NAS name appeared in the Shared Folder field again!  When I left, and wnet back to that page, the Shared Folder field again showed the new NAS.   For background, the old NAS is still on the network as it has some video content that I am still able to access from my Smart TVs via DLNA

Do you have a PC or laptop with the desktop controller installed? I suspect the old NAS won’t show at all in the Music Library Settings, or when you initiate an old fashioned ‘Update Music Library Now’. 

I wonder if in the new mobile/tablet apps the term ‘Scan for new Content’ means it’s doing more than the old app and actually looking for other stuff on the network, seeing your old NAS, and then realising it doesn’t support SMBv2.. I’ve noticed a few scenarios in the new app where it goes through some ‘catchup’ which triggers UI animations. This could be something like that.  

Yes I have a Windows app installed and it always shows me the Music Library perfectly, apart from the indexing problems which are REALLY ANNOYING.  For example, I have one album which was ripped/imported in one go, but 3 of the tracks show up as ALbums with the same title and one track, whilts the rest of them correctly show up as all part of the same album.  I’m beginning to fear the worst in that its been such a long time now that I’m part afraid that Sonos are goigng to be unable to fix the indexing problem they created 

The indexing issue should be fixed on the latest firmware. Also, I’d check in the Windows App in advanced settings for the music library that you have the correct group albums by setting. It’s only in the desktop app and missing from the new ones. Obviously it needs to re-do the index once on the new firmware.