I have verizon and use this https://www.vzwpix.com/login.do to upload, there is a 3mb cap on uploaded media as well as I think 60 seconds per audio file and you can only send them in these formats
Images: .JPG, .GIF, .WBMP, .BMP, .PNG
Audio: .AMR, .QCP, .WAV, .MIDI
Video: .3GP, .3G2
Edit: There are mp3 to wav converters and then you can use the built in windows wav application to chop it down