Hey Mark,
The best thing to do is check what you have set for the recording preferences for the input for the sample and bitrate. If I had to guess, I'd say the sample rate is a mismatch for the recording vs. playback. If you can click on the file in rekordbox, then open the info panel to the right to show the details about the song, send a screenshot of that. Next, take a screenshot of the audio configuration from the rekordbox preferences. Post'm here and we'll see what might be going on.