In WordPress, there’s the option when you’re adding or editing a post (or page) to “Add Media”. This may look long-winded but it’s probably as quick to do as it is to read:
* Click at the place in the post where you want to add the MP3
* Click that button that says “Add Media”
* Click “Upload Files”
* Drag & drop the MP3 or locate it after you click “Select Files”
* Change the title that’s (usually) automatically generated if you want/need to.
* Check the “Link to” box says Media file (it probably will)
* Click the Insert into post button
* Click the link that’s been put into your post – there’s the option to delete it or edit it, select the edit option
* Click the link options cog wheel
* Tick “open link in a new tab” & click Update
* Publish/update the post
The MP3 would then be hosted on your website.
You’d deliver them by sending people to that page whether that’s in your autoresponder or somewhere else.