I first got this phone as a replacement when my LG Muziq broke. I was considering the M520 before getting the Muziq, simply due to the fact that both phones had an MP3 player and supported the Pandora application.
At first, I had problems getting the Pandora app to even connect on the M520. I tried to start up the program every few hours, but it still wouldn't log in. I tried uninstalling it and then reinstalling it, but still no go. I checked to see if the firmware on the phone needed updating, but no updates were available. I took the phone back to Sprint, where I was told they would essentially refresh my connection to the network and that Pandora would run in a few hours. The good news is that Pandora worked, the bad news is the headp... (continues)
I'm so sorry you're having this trouble. I've had an m520 since last May, and I haven't had any trouble with the headphone/charger jack. I did have a couple of other glitchy issues (I'll post in a minute...don't want to hijack your thread). I have noticed that the volume when I'm just using a single-earpiece headset for making calls isn't what it should be...but only when using the voice dial. I'm looking into getting a stereo Bluetooth headset so I can use Pandora while charging...maybe you should think about that, too...
P.S. Pandora is now free...so say goodbye to the $2.99/mo charge!