While looking at other possible providers, I came across this in the Lyrion/LMS Mixcloud plugin notes: “Note that although Mixcloud has a public API to browse mixed and get metadata, Mixcloud does not have an API (or any open method) to stream mixes outside of the website or the app. Therefore this plugin has to rely on other methods to get the URL to stream the mix. In the past many online services ("Mixcloud downloaders") were used to get this, but they might stop working at any point. As of verion 0.11 a helper application ("yt-dlp", a fork of "youtube-dl") is used.”. Ergo, it depends on an unofficial workaround.
The Mixcloud API documentation link posted earlier also says “The audio streams are not available through the Mixcloud API. There are two reasons for this. Firstly, we need to know what has been listened to so that we can report usage, pay royalties and provide features such as 'Suggested Shows'.Secondly, Mixcloud needs to pay the bills! We can't give away the audio for free outside of mixcloud.com simply because it costs us to host and stream the files and pay royalties.”
As with other services, Sonos may well have negotiated a way around this to include it in their app, but I doubt if Wiim’s pockets are as deep.
Also, I keep coming back to why users think the WiiM Home app should be the be-all and end-all app supporting all music services out there. Even when third party APIs are available, they’re often if not always less functional than native apps. And when it comes to services with lossy, lower audio resolution (like the 320kbps I believe Mixcloud offers), then sending from the native app over AirPlay, Chromecast and, God forbid, Bluetooth, can be more than sufficient.