You need to add an offset to your mp3 by using an external program of audio editing software such as Audacity, Adobe Audition etc etc.
But as for me though, I have my own way to add it but I'm not using the programs that I have mentioned. I'm using BMX2WAV as the main program for me to convert from .bms to .wav. After that, convert again from .wav to .mp3 by using any audio converter software. As a result, the .mp3 that I want is working greatly than picking .mp3 from Internet. Oh and, at .bms file, you have to rearrange the BGM so that the .mp3 output will be working nicely as it gets.
So, As for you though, it's better to use Audacity or any audio editing software that you have got in your PC (if not, get one) and add the offset to it. As how to add to offset to it, watch the video here, http://www.youtube.com/watch?v=tnts9Rzw1GM
By the way, offset = blank/silence sound at the start of the song. haha