Assuming you have installed your graphics card drivers correctly, you will still want to quickly verify your HDMI audio out is working before any further mucking about in the operating system environment. In Ubuntu 14.04, this is done quite simply from the menu bar.
Click the sound icon in the upper-right corner, then in the context menu that appears, click on “Sound Settings”
In the Sound Settings dialog box, verify that your sound card is activated and click the “Test Sound” button.
Click the test button for each channel and verify the output.
Other derivatives of Ubuntu (particularly the lightweight Lubuntu) do not have the robust GUI that Ubuntu features. In these cases, a little terminal jiggery-pokery will be necessary.
Verify the HDMI audio output with this terminal command:
aplay -D plughw:0,3 /usr/share/sounds/alsa/Front_Center.wav
Use Nano (or another inline text editor) to add the following line to /etc/asound.conf AND/OR ~/.asoundrc (depending on what your distro uses)
pcm.!default = pcm.hdmi
Reboot, and you should be up and running with full HDMI stereo sound!