I had a bit of trouble getting my speakers to work, but I found a quick solution to get them working. Just in case anyone else has any problems, just follow the steps below:
1. Plug in your usb speakers.
2. Click the Mint Menu at the bottom left hand corner of your screen.
3. Navigate to Volume Control (PulseAudio Volume Control).
4. Click on the Output Devices tab.
5. You should see USB Audio DAC -USB Audio device listed. Next to the lock icon there is a disclosure icon. Click on it and choose the default checkbox.
6. Close window and open a movie or song file and you are set to enjoy your speakers.