How to add an emoji at the end of several WAV files without renaming them all?

Select all of the .wav files in the Finder, and then right-click on them. On the secondary menu, select Rename… and use this setting with your preferred emoji:

and click rename on that panel. The emoji is appended to the filename prior to the extension.

You could use the rename Action in an Automator workflow to add text to the end of the names.