Thank you for reaching out here with your question on embedding an MP3 in an email! Unfortunately, there's not a way to do so in Constant Contact, and most email clients will not support embedded audio, particularly if it autoplays. You can link to an audio file if it is hosted online, as you would any other regular weblink, however! Here is an article that might help. If you have any more questions or concerns, please don't hesitate to ask.
Thank you for pointing that out! It looks like that link was to a help article that is now outdated. The process is still possible though, and the updated link can be found here. I updated the link above as well.