I have on several occasions gotten an error message on my blackberry stating "can not play media while alert is active". I have gone thru the full menu screen checking that there are no email, text, bbm, twitter, google voice or other alerts. Generally I never find any active alerts, and I am not about to reset the phone to fix this problem. I have found a quick trick to get rid of this problem. What is the main function of a cell phone? To make phone calls - so making a phone call should take priority over everything else, and apparently it does. Even if you do not have a signal just attempting to place a call should allow you to playback your media without having to do a reset.
Sent from my BlackBerry® by Boost Mobile
Update: Aug. 27, 2011
Apparently this does not resolve all "can not play media while alert is active" problems. While most of the time that I get this error message the above has worked to clear that message and allow me to play media with out a battery pull, I have since posting this had several occasions where this information does not clear this message. However I still would recommend giving the above a try since it takes less time than pulling your battery and waiting for the BB to reboot.