Steps to reproduce:
1. Create a project where audio file is played once GUI.Button is pressed
2. Deploy and run it on a Windows Phone 8 device
3. Press the button to play the sound
4. Lock the phone by pressing lock button
5. Press lock button again and slowly slide screen up to unlock the device
6. Audio playback will not resume and pressing the button will not play it again.
7. Re-lock screen and unlock it in a faster manner
8. Audio will be working again

Not reproducible with
Version 4.3.0f3 (7eed886c2a59)
Wed, 06 Nov 2013 21:12:33 GMT

In order to fix this issue, device should be updated to Windows Phone Update 3. Addotional information can be found here: http://www.windowsphone.com/en-us/how-to/wp8/basics/windows-phone-8-update-history