For those interested, this is an old post, but a lot of it still holds true:
https://lameguy.wordpress.com/2013/02/08/subsonic-install-in-freebsd-jail/
For the jail, I just set up a basic jail. I only modified the IPv4 address to the one I wanted on my network (DHCP would work too, I'd bet).
Some of the packages have different names or newer versions than in the tutorial above, and you'll want to get the newest version of Subsonic (6.1.6 for the standalone version at time of writing).
To get it to autostart subsonic when the jail starts, I added sh /var/subsonic/standalone/subsonic.sh to the top of /etc/rc in the jail. That script is set to run at jail start (exec_start) by default.
I already knew how, but you'll also need to know how to mount the music files you want to the jail. It's pretty easy.
The rest is just Subsonic setup. I like mine to run on a specific port; that means editing the subsonic.sh file, but that's easy enough.