Hooray for Time Machine. In an idiot move I deleted an Audio Hijack 'Session' instead of a recording.

I found the sessions file at '/Users/miraz/Library/Application\ Support/Audio\ Hijack\ 4/Sessions.plist' and restored an older copy.

Sorted!

Turns out the session was simple …

The session shows the Safari application sends an MP3 file to disk and plays the sound on my Mac.