Sounds like a network issue. Try the troubleshooting tips in this article:

https://support.sonos.com/s/article/4725?language=en_US

All of my iTunes music plays without issue. I did, early on, have to re-download some music, as some of my files were old enough to have DRM applied to them. But since that point, years and years ago, I’ve not had any issues.

The fact that those files still play for you later suggest that certainly you don’t have the same issue that I did, but points to a network issue, most likely wifi interference of some type. The unfortunate fact that sometimes interference comes from outside of your network is indeed distressing, but just requires some simple testing.

One suggestion that Sonos doesn’t frequently make is the assignment of reserved IP addresses for their speakers. I’ll admit my old router had, for reasons unknown, a hard time keeping track of already used IP addresses, so any time Sonos updated their software, the reboot of the speakers would cause my router to hand them ‘bad’ addresses, and then I’d get something very similar to what you’re describing. It took some research in the manual for that router, but after figuring out that process, I stopped having issues. When I replaced that router, I continued with the reservation process for all of my network devices, and left ‘visiting’ devices to use unreserved IP addresses. 

Thanks. I only have a “Sonos Connect” unit  that is wired to my stereo with no other Sonos products and it is all hard wired to the router, not using any WIFI. That is why I think it is a software problem with the Sonos S1 app. 

Might be time, when it happens again, to submit a diagnostic and call Sonos to see what it says. 

I will just assume you are streaming from an online (reliable) source using a high speed Internet connection as per the Sonos requirements. However If the ‘Connect’ is wired direct to the Router, then try a different cable. If it’s wired elsewhere on the LAN, to either a ‘switch’, or other ‘access point’, then perhaps cable it direct to the router instead and cut out any ‘middleman’ and maybe see if those things solve the matter for you.

Thanks. Am actually playing from  imported playlists from iTunes which reside on the same PC. My Sonos unit is a Zone Player 90. After songs play for a while i get and error message that the song is unable to play. It might even skip a bunch of songs (that have played successfully before) saying for each that they are unable to play.  It would seem to me that problem would be between the S1 software and the iTunes library and not the Zone Player since it plays whatever the S1 app sends to it.

The S1 App is not a player, but just a controller (remote) for your speakers, so it’s likely the connection between the PC and the Speaker - perhaps some element of ‘power saving’ on the PC is  causing the issue?

I would maybe check the power saving options for the PC HDD and Network Adapter etc; to see if they maybe enterering a power saving mode after a certain period of time.