Yes, you can attach .mp3 files as an attachment to a single email campaign. If you are on the Core Plan, the file will need to be 1 MB or smaller. If you are on the Accel or Enterprise Plan, your file will need to be 5 MB or smaller.
If your file does not meet the size requirement, you can host the file elsewhere and link to the file within the content of your mailing.
Everything you need to know to attach your audio file can be found in this Knowledge Base article:
If you have any questions, please don't hesitate to let us know. You can find all of our great support resources and how to get in touch with us via email, chat or phone on our Support Page.