Thanks for your help all! In answer to your questions:
ctml Is there anyway to validate this was not perhaps just a change on the backend by soundcloud?
Just to clarify, if I share the same private link on my Wordpress website the embedded player is created in the same way it used to be on Flarum.
My students are sharing their 'work in progress' tracks, so they share them for feedback but not so the public can see them until they're released by the record label.
clarkwinkelmann Can you share a link to such a private playlist so we can do some tests?
Here's an example - this link creates an embedded player perfectly on my Wordpress website, however, pasting it into Flarum creates this:
clarkwinkelmann Have you tried going back to a previous Flarum version to see whether it's Flarum related or indeed just a change in Soundcloud?
I haven't, I'll be honest - I found it quite tricky to upgrade from beta 12 to beta 13 so I've just stuck with that ever since, and wouldn't like to attempt downgrading. Flarum has been a steep learning curve for me!
clarkwinkelmann If Soundcloud changed their policy regarding embedding private playlists, there's probably an article to be found on their website about the change.
The same private playlist will happily embed anywhere else I've tried it, just not on Flarum anymore.
tankerkiller125 Looking deeper into this it appears to be a soundcloud permission issue. Your have to make sure that the Display Embed code option is enabled. They likely changed it to be turned off by default recently.
The permissions on Soundcloud are set correctly and an embedded player is created when the link is posted in Wordpress or other forums, as it used to on Flarum.
Thanks again for your help, much appreciated.