Want to Make Itunes only play when I explicitly unpause or start it
"This afternoon I received an email with an audio attachment, which I played."

Did you double-click it, or Open, which would open it in iTunes?

Once you've set iTunes going it will then follow its pre-set behaviour, which includes playing the following song.

If you don't want to bother iTunes at all with the audio file, you can simply select it &amp; tap Space Bar, which will make Quick Look play it instead. This won't touch iTunes, won't add it to its data storage, nor will it then play anything else once it's finished.
You can start/pause or change playback location. Space Bar again will close it.  

The only thing you can't do is select something else in the same app whilst it's playing, or it will Quick Look that instead.