There are two parts to this.

The client you use to compose the message needs to be able to insert the media.

The client used by the recipient needs to be able to play embedded media.

So, even if you can find a way to create such messages, there is no guarantee that the recipient will hear it.

Please consider if an unexpected belch of noise will cause your correspondents any distress. I'd be very embarrassed (and angry!) if it happened to me at the workplace.

The short answer is that Thunderbird is not suitable for your requirements.

As Zenos doesn't know uder what circumstances I intend to use this I find the reply both curt and unhelpful. It doesn't inspire confidence in Thunderbird when this is something that I seem to remember receiving and sending some 15 years ago with a very early version of Outlook Express - All I want to do is get away from the microsoft monopoly.

Yes, Microsoft email programs allow, or used to allow, the insecure practice of running embedded scripts. They have since improved their approach to end user security.

I don't want email to be able to run programs on my computer without my express permission. So I use Thunderbird and not Microsoft email software.

I have seen an example that allegedly allowed the system's default music player to be launched from an email message. I am not sure that it would work if the message was viewed in Thunderbird. It is non-trivial for the sender to set this up (you need to insert some code).

So the answer remains that Thunderbird is not suitable for your requirements.

There is an example here: https://www.campaignmonitor.com/blog/email-marketing/2012/12/adding-audio-to-email-newsletters/ but a) I don't think Thunderbird supports html5 (and if not, it won't play the music) and b) note that in this example, the audio is obtained via an URL linking to a web server; it isn't embedded into the message.

Attachments are sent by being encoded and appended to the message text, and internal links created to allow it all to be unravelled. While this is supported for certain types of images, which can then be displayed inline within Thunderbird, there is no formal support for encoding and decoding music in the same way.

There is nothing to stop you sending an mp3 file as an attachment, but no support in Thunderbird to allow it to be played.