How can I send mp3 files via email to my friend free of cost

Yes, just send it a an attached file.

Why would you think there was a cost involved? Have you ever paid anything for e-mail? Generally, you pay for an internet connection. After that e-mail is generally included and there are many free services.

There is a caveat here, in that, if these MP3s are of copyrighted commercial songs, then sending them to a friend would be considered a violation of copyright laws and illegal.