Why is my music stopping when I put my android to sleep?

[AC Question]

My music suddenly stops playing whenever I put my phone to sleep. Whenever I turn it back on, the song continues from where it stopped. I also found out if i'm playing a song and put my phone to sleep, if I put the volume up the song continues like normal in sleep mode (but I have to put the volume up every single time I put my phone in sleep mode). I have an Alcatel One Touch Fierce and I've had it for quite a while now and haven't experienced any problems until recently.