With the Mathematica Kernel on a desktop computer, you would not see
a user interface open; it would happen without opening a window.
If X-Windows is not running on your Raspberry Pi, then the things that the Mathematica Kernel needs
the Mathematica Front End for will not work.
Other suggestions about sound with Mathematica on the Raspberry Pi are at http://community.wolfram.com/groups/-/m/t/161434