On the web there are tons of websites providing nice greeting cards that can be emailed instantly. You can also find music greeting cards that includes embedded audio which can be used for different occasions. And if you want to custom tweak the greeting card in your own style then try out Postcard which is a online service. Postcard will help you to create custom email greeting cards in which you can listen to audio music of your choice as a email message. Here is one custom email greeting card I created in a couple of seconds using Postcard.
All you need is a photo or any attractive wallpaper and upload your favorite Mp3 audio song. Include the email id of your friend and browse around till the mp3 song file is uploaded. You can also preview the custom greeting card you have created. The service is free to use and is useful on many occasions