As mentioned above, just about all e-mail providers limit the size of e-mails. It varies by provider. (You'd need to contact your e-mail provider to find the maximum permissible size they allow). Putting your video online and providing a link to is the best way to share vids.
However, if you have a short MP3 clip, say 10 - 20 seconds, the file size it might be under the limit, in which case you'd be able to send it.
To send an MP3 file, you use the 'Attach' function of your e-mail software. Without knowing which software you are using, I can't give a specific answer since the methods differ.