if you have access to cygwin tools (or a linux/unix machine)
dd if=in.mp3 of=out1.mp3 bs=1048576 count=512
dd if=in.mp3 of=out2.mp3 bs=1048576 count=1000 skip=512
(the second count is higher since I assume your file isn't exactly 1 GB)
to put them together again: cat out1.mp3 out2.mp3 > new.mp3