Yeah library not appearing in sources seems to happen for a lot of people. For me force quit then reopen the app, sometimes 4 or 5 times in a row will make music library appear. YMMV. 
 

There should be an update to the app for local libraries this month. Whether it fixes any of the many bugs or just adds features remains to be seen. 
 

If you are on iOS you can trash the buggy AF Sonos app and use SonoPhone ($3), it works every time for me. Or go back to 16.1 version if you have Android. 

What NAS is it and what SMB (min/max) version are you using? - Does the NAS configuration pages list any of the below features and if so, what are they set to currently?

• Oplocks

Write Options:

• Preallocate
• Strict Sync

Security:

• Restrict Anonymous
• SMB Encrypt
• SMB Signing

This is a discovery issue, if you know your way around your Router check the  settings to ensure Multicast traffic (mDNS) is allowed. Also see if there is a setting that says IGMP Snooping or IGM Proxy... turn it on.

Worth a try but with so many ongoing issues it hard to pinpoint whats going on.

Edit: Wiim as well as Bluesound both have Port replacements.

Just to add. 
this is a Sonos system that’s been active and running for as long as SONOS had been in UK. 
this NAS was fully accessible and active on old app.  
 

iIt’s the new App that’s caused this. 
or are there new instructions for the new App and NAS access ? 

Yeah. I have the Blusound equiv in the office.  May swap them.  

Your NAS needs to be sharing in smbv2/3, not smbv1. See here on how to double check that your nas setting is correct

https://kb.synology.com/en-ca/DSM/help/DSM/AdminCenter/file_winmacnfs_win?version=6

All of that article is about the clients accessing the NAS and not about the NAS.  The NAS settings have been working for Sonos and Blusound for years.  Something in the new App is causing this. 

Sorry I’m not a synology owner thought it had instructions to set it to smbv2 in the device itself which you don’t need anyways ‍♀️

100% the new app has issues showing the library link in sources reliably IME. This is a known issue. 
 

Switching to a different app like the Sonos desktop or SonoPhone (iOS) are the only ways this works for me reliably. 

That article details an assortment of settings for Synology NAS.

Sonos added SMBv2 some years back and left SMBv1 working; in other words you could use either. In early May 2024, kinda-sorta simulataneously (within a week), both the new mobile app and the device firmware stopped supporting SMBv1. However (!) we’ve seen users encounter issues if their NAS offers SMBv1 and SMBv2, and in those instances disabling SMBv1 on the NAS fixes the issue.

Update to question( couldn’t edit the original)!

So after a morning playing a queue from my NAS I played nothing for a few hours. 
I opened the app again at 17:22 and noted that “My Sources” (the NAS) no longer existed and the Queue was empty.  At 17:34 the Queue was now showing the mornings tracks and I can play them.  But still no sign of MY SOURCES.  It’s now17:49. 
still no sign of my library. 

NAS access is clearly OK since it’s playing the previous Queue loaded this morning. 

Who designed this app. It’s the worst app I’ve ever used on an iPhone.  Staggeringly bad.  Really hard to understand what sort of spec they were working towards other then “get rid of every feature you possibly can and strip it down to being dependent on the other providers apps” .  Ie. Spotify et al.  

I’m assuming the auto update of library index feature has gone. 

ho hum.   I’m sure it will find the NAS eventually. 

 

I have reported issues to Sonos with the queue played from local library, load queue, play, stop, resume playing a while later and either the queue position resets, the queue gets wiped or nothing happens. Unfortunately it’s not reliably reproducible.

Check your router as per my earlier post,

I thought I knew my way,  but nothing in Router shows any of these.  BT Hub. 
advanced settings shows nothing.  Where would these features likely be in Router sections.  
meantime it’s playing the queue happily. 
 

Unfortunately you have one of the most abysmal free device ever given away, Last Hub BT sent me never even got out the box. Do have a search here as there have been a few posts, Disabling the 5Ghz SSID has helped some in the past as does turning off Smart setup.

The BT Hub does support multicast but there are no settings I am aware of, the Smart Hub 2 does not support it at all.
 

Ironically Bluesound also suffers issues with the BT Hub range.

Edit: multicast is used for discovering device that use Samba aka NAS devices.

My “abysmal” free device has served all my network and broadband requirements for 20+ years.   Many thanks for your input on that. 
Sonos has been running well via these abysmal routers since the year they (SONOS) arrived in the UK. 
It seems as if  you are guessing with these answers or do you have a specific reason why these features are now required as a result of the new app when they’ve never been required in the older apps.

Is there a new tech requirements doc from Sonos?

Blusound works fine.  I have it in the rest of the house.   
 

Sonos changed the way devices are discover back in May, using a different network protocol.

Sorry I am unable to help you with your problems any further.

British Telecom Routers have a bit of a checkered history, especially with network multicasting device discovery, it seems…

In some cases it was recommended to use wired mode to get around various discovery issues, but that’s really problematic nowadays as newer Sonos products and all portable Sonos Speakers do not support that mode. I also see that their BT HomeHub v3 was placed on Sonos’ hardware incompatibility list here:

https://support.sonos.com/en-us/article/incompatible-network-hardware

For most users if what they were using was compatible with SSDP it should be good with mDNS - but there will be edge cases where this isn’t true. 
 

Likely if OP switched to SonoPhone or the desktop app (both use SSDP/UPNP not mdns) the issues will disappear as the new app is bugGY AF.