The Monkey’s Audio format (.ape) it was perhaps the first audio compression format lossless (without loss of quality) in becoming popular, but due to license restrictions and the development of better alternatives, has been superseded and forgotten in Linux, particularly in KDE applications: Amarok cannot play it since it stopped supporting GStreamer, and K3b requires a plugin that may or may not work with the latest version (I didn't bother to compile it).

Fortunately, and against expectations (my search results consisted exclusively of hand-run scripts, all with half a dozen dependencies), rescuing an .ape is very simple once we convert it to FLAC, as long as we have the index (.cue).  To do this, just use FFmpeg, and then K3b, which includes a practical (although not perfect) format converter.

The following commands are in charge of converting the .ape to FLAC, create a new .cue, and open it in K3b. You just have to replace path / to / file.ape for yours, and run them verbatim (one by one, or hitting all of them at once, anything goes).  The .cue is assumed to have the same name as the .ape.

APE='ruta/al/fichero.ape'
ffmpeg -i $APE -acodec flac ${APE%.ape}.flac

perl -pe 's/(FILE.*).ape/$1.flac/i' ${APE%.ape}.cue > ${APE%.ape}.flac.cue
k3b ${APE%.ape}.flac.cue

Then it remains to convert it to the format of our preference in K3b.