How to add Audio files to Photos?

It is not possible. You can only add photos and videos to a Photos Library.

If a Photos Library contains audio files (for example, if you converted an Aperture Library or an iPhoto Library to a Photos Library), the audio files will be causing syncing problems in the Photos Library.

This help page explains, how you can import photos and videos to Photos for Mac: Overview of importing photos and videos into Photos on Mac - Apple Support

The Music.app (or iTunes on older systems) would be a better place to manage your audio files. Any app that needs an audio file can use the Media Browser and the Media Browser will show you the audio files in the Music.app. For example, if you want to create a slideshow in Photos, look for your songs in Music.

Yu could create videos from your audio files in iMovie by creating a video from a still frame with the audio file as a sound track, but then you can no longer use it as a sound file. On a Mac we have separate places for photos, audio, and videos.

• You keep photos and short video clips in Photos for mac
• Audio files and songs in Music (or iTunes on older systems)
• Home Videos in TV
• Books in the Books.app

The Media Browser will show you the items in the corresponding apps, when you need them.