OK managed to fix this; you will need to use the following settings:
On the zentyal admin page browse to Users and Groups > LDAP settings
On the advanced page of the subsonic settings you need to have the following from the zentyal LDAP settings:
LDAP URL: ldap://localhost:390/<Base DN values>
LDAP search filter: (uid={0})
LDAP manager DN: <Root DN values>
Password: zentyal LDAP password
The key is getting the password from the zentyal admin interface, I discovered that if you simply cut & paste it an additional [space] gets added at the end, I used the value from the webmin ldap interface but you could simply paste it into a text document first.
You will need to open the port used by subsonic on the zentyal firewall