EDIT: I found this page http://www.subsonic.org/pages/database.jsp detailing the external DB setup ...
I searched forums to use mysql and found nothing, so I am posting what I have done on Debian to use a mysql database.
edit /etc/default/subsonic and add --db=JDBC:mysql://localhost:3306/DATABASE?user=DATABASE_USER&password=PASSWORD to SUBSONIC_ARGS= section of file and restart Subsonic (service subsonic restart) and db schema will be created and you should be good to go ...
Have a good day ...