64MB is all of the phones memory, which includes the OS, the storing of data, the storing and usage of applications.
You have say after a regular installation of OS and no data maybe 32MB left, and you haven't even setup email, got any messages, inputted your contacts etc etc
Theres no way you'd fit 50 MB's of MP3's onto that, and actually have your phone run for very long.
Get a media card now
"That's it then. Cancel the kitchen scraps for lepers and orphans, no more merciful beheadings... and call off Christmas."