There are two ways
This is the safest method:
Upload your MP3 file to a publicly accessible URL (your website, CDN, cloud storage with a direct link).
In Stripo: add a Button or a “Text with link” block (or image + link) that says something like “Listen to audio” or “Download MP3”.
Link the button/text/image to the MP3 URL (or to a page that plays it).
Optionally add an image (album cover, audio icon) for visual context next to it.
If you want to attempt an embedded audio player:
In Stripo, add a Custom HTML block (Stripo supports HTML blocks as one of its basic blocks).
In that HTML block, insert markup like:
<audio controls>
<source src="https://yourdomain.com/path/to/file.mp3" type="audio/mpeg">
Your browser does not support the audio element.
</audio>
Set up the source URL (src=…) to the MP3 you uploaded.
Then test the email in multiple clients.