Ok. Sorry to do it this way. But i need to be at the same point you are to troubleshoot so here are some basic questions.
1) Have you tried hooking up your phone to your pc with the pc connect cable and downloading directly to the phone, ergo circumventing the sd card?
2) Have you tried to use an mp3 file with the extension .3g2, so effectively removing the mp3 or .qcp extension and making it .3g2? for some reason this tricks the phone into thinking it's a file native to the LG. No conversion programme or anything needed, just change the ext
3) Are you a virgin mobile customer? (Virgin the company, don't go getting crazy ideas) If so, Virgin uses weird security protocols, so if the above didn't work I need to know this before proceeding.
please try these steps if you have not already, if nothing works let me know and I'l help you along.