it depends on your mail provider, but you should be able to attach any file you want to your letters.
The compression on mp3 files are quite high to begin with, but you can still use a compression utility and cut the size of each song file by a little bit.
If size is still a concern and quality isnt so much, then try to reencode your files to 112 kps or even 96 kps. Will be about 3 times smaller than a 360 kps encoding.
Just look for the attachment option, usually a paper clip, browse your harddrive for the song file, and attach it.
If you have access to any FTP server, then I would highly recommend uploading the file to your ftp server, and then email the direct link and have it downloaded remotely from your ftp server. It will save you time, and since email is sent in an older format, mime I believe, you will notice that letters are always larger than the files you attach, so this wastes alot of time to upload the letter and for it to be downloaded on the other side. So if you use the ftp server, then you are actually transmitting just the file itself so its a smaller upload and download.
Cya
BigMike
