Raspbmc audio Scarlett 8i6 I/O

I am trying to get RaspBMC to play audio through a Focusrite Scarlett 8i6 soundcard but so far I cannot play any music through the default interface. I have tried checking the external audio interface's option in the normal RaspBMC menu. I managed to successfully play a .wav file using this command line input (this was just a random .wav file I found online). aplay sample.wav -D sysdefault:CARD=USB Trying the same command with an .mp3 file did not seem to work. Any knowledge of how to get the audio input working would be helpful too.