There’s potentially several reasons for this, and your post doesn’t contain enough information in it to be certain.
I would try four things:
First, refresh your Sonos network, by unplugging all Sonos devices from power. While they are unplugged, reboot your router. Once the router comes back up, plug back in your Sonos devices.
Second, read through the wifi interference FAQ, and check for any potential issues that might be covered in that documentation.
Third, check to make sure the files are stored as a type that Sonos can read, as discussed in the supported audio formats FAQ. If they’re stored in an incompatible format, they would be skipped by Sonos.
Finally, if none of those apply, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, or post the number here.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis.
These tracks are not stored. They are streamed. Like spotify. Spotify works but mixcloud does not. Except when streamed direct from the website
That doesn’t alter the need for them to be in an appropriate format. Stored or not, there are restrictions as to what file types Sonos can play.