Check the meta-data on each problem track, it likely isn’t matching the file name.

Ah Stanley, if only I knew how to do such simple sounding things!  I use Sonos to play music and have no idea what goes on under the bonnet, so could you kindly give a little more guidance please? Thanks, Ian.

Look for a track editor or tag editor for your operating system it should have instructions 

 Are you Windows or Mac?  MP3 Tag is a good tagging software.  Don’t know if it’s available for Mac.  The track number has to be shown in the tag for the tracks to be shown in the correct order.

Thanks Stanley & MoPac. I’ll see what I can do.

But, as an aside, why might this have started happening when it hasn’t before?

Ian.

 The track number of the track is not what is looked at when software sorts the order of the tracks.  It’s the track number in the tag that determines the order of the tracks.  So if there is no track number in the tags of each track they will be sorted alphabetically.

 Not sure how you’re acquiring YouTube music, but likely it is not tagged.  So each track would have to be tagged using something like the tagging software MP3 Tag.  Highlight each individual track and fill in only the Track Name and Track Number.  Then you highlight all the tracks in the album and tag Artist - Album - Year - Genre - Album Artist.

 For a compilation the Artist would be the performer and the Album Artist would be “Various Artists”. 
 

 The category names may vary depending on the tagging software used.  If you wish you can also fill the Composer tag.  I never use that one but many folks do.

MoPac - Thank for that.  I’ll give it a go when I’ve some time. Ian.