My guess is that the soundtouch cannot see the My Cloud Home private user space. the My Cloud Home is not a traditional NAS. The Z: drive is a private user space and is not a traditional network share like most NAS devices use.
There is a default Public share that you may be able to use so the soundtouch can see the music. If you use the default public share however you will not see the files in the Z: drive, in the My Cloud Home mobile app, or in the My Cloud home web access.
The article below is for sonos devices but will liklely work for the soundtouch as well.
https://support.wdc.com/knowledgebase/answer.aspx?ID=20694&s=
If you do not want to use that method you could return the My Cloud Home and get a normal My Cloud device.
