The audio sequel is a much more satisfying continuation than Echoes. Not that Echoes is bad but it got cancelled after one season thanks to its mindnumbingly dumb release schedule.
Echoes is also set decades in the future so there isn't that strong of a continuation. Middle-aged Kira played by Keeley Hawes is one of the main characters, and Delphine and Felix cameo in some episodes too, but that's it