Problem playing SD media card

[CWP8]

We just entered the world of Mercedes and very happy with the car (2018 A200) so far (we've had it all of 7 days). However, we are having trouble with the Audio20 Infotainment system. I have created two folders on a SD card: one for Music and one for Audiobooks. While the audiobooks, of course, need to play the chapters sequentially, we often like the music to shuffle/random play. When I set the playback option to Random track list, yes, the playback shuffles...but it doesn't just shuffle the tracks in the Music folder but all tracks on the SD Card, with the result that one minute, we're listening to a song and then it will jump to say Chapter 5 of an Audiobook. Very frustrating as you can imagine (esp for our 8yo and 5yo kids!). I've probably spent hours googling and reading the instruction manual, but can't find a fix. On an Audi forum, one user said his solution to essentially the same problem was to set the relevant folder to repeat play and then shuffle that folder. But I can't find a repeat folder (or, for that matter, any repeat track ) option on my system. Any ideas gratefully received!