I have tried installing several music Apps but none of them see my USB containing music files whilst Zorin Pro does see them all under "other locations" Any advice as it appears to be a Zorin Pro configuration issue
Are the music apps installed as flatpaks? If yes, they are sandboxed from the system and have no access to your files.
Install the app flatseal and then open it and select your music apps and give the permissions to your music folders (at category filesystem).
The Apps I tried were Clementine, Strawberry and Deadbeef - are these flatpacs as I installed them by searching and finding them from the Zorin Software App
I am new to Zorin so have a lot to learn
In the software center you can look in which package format (Zorin apt package, snap or flatpak) a package is installed. Select your app and under the "open" button is a field with an arrow. Click there and look which package format has a 
.
If it shows flathub/flatpak, then go to the app flatseal (I think it is preinstalled on Zorin Pro) and change the permissions to give access to your music data folders (add the path).
Often, apps are available in various package formats. With the Zorin apt (=.deb packages) you have less problems because they are not sandboxed.
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I am dependent on specific music app as I have a SoundBlaster AudigyRx sound card with Logitech surround sound system, so I use the best for me Audacious, and qmmp as an alternative.
For clarity I don't run ZorinOS anymore.
usb sticks usually show under /media/$USER and flatpak strawberry cant see that path till u allow it
-findmnt | rg -i 'media|mnt'
-Flatseal → Strawberry → Filesystem → Other files → add /media and /mnt
-or install the deb instead:
sudo apt install strawberry
reopen the app after