Audio Stops when screen goes black (not just locked)

Hi, I've found a problem that the background audio goes silent anytime the screen goes black (as in locked screen black). The funny thing is that it starts and stops based on the screen being black or not, even if the phone's still locked. To see what I mean, using the Tutorial 9, build and run, then lock the phone. Then push the button on the top left of the phone and the audio starts again, even though the phone is still locked. As soon as the screen's black again, audio stops. Is there a way to fix this? Is it a known limitation?