is it possible to add a sound effect to an imessage? For instance, a bell ringing or a horn honking, to the message itself?

Greetings Mblanco59,

That's a great question! You can add effects to iMessages in by following the steps listed below. These visual effects can include sounds as well. As for being able to choose just a sound, that isn't supported. Apple appreciates all feedback regarding their products, services, and features. If you would like to provide this feedback for Apple to review, you can do so via the Product Feedback website.

1. "Open Messages and tap the Compose button  to start a new message. Or go to an existing conversation.
2. Enter your message. 
3. Touch and hold the Send button , then tap Screen.
4. Swipe to the left to see full-screen effects.
5. Tap the Send button ."

Use message effects with iMessage on your iPhone, iPad, and iPod touch

Take care!

You can share a file from Voice Memos

Share a recording in Voice Memos on iPhone - Apple Support

So, I guess you could record a message with the sound in it.

Bit of a kludge, I grant you.