Solved! Finally!
Today I played an audiobook ripped from a CD and copied onto my USB drive and the tracks played in correct order for the first time after years of trying. I did one thing different this time which seems to make the difference: I ripped to mp3 format instead of wma format. This gives credence to those who thought it was the
ID3 tags that determine the order.
I had tried converting wav or wma files to mp3 before, and they still wouldn't play in order. I would guess that's because the ID3 info wasn't in the original wav or wma files. So I ripped directly from the CDs to mp3 this time and checked the properties and there are many ID3 tags shown, including one labeled #. I think that must be the one used by the Leaf player.
So why didn't I ever try this before? I didn't know how. I'd always used Windows Media Player to do the ripping and the mp3 option wasn't there. WMA was, and that's the only other format the Leaf player can play, so I used it. I never knew that you had to download the LAME codec from sourceforge or elsewhere. Apparently due to copyright or patent issues programs like Windows Media Player and Audacity can't include that codec in their programs. I had a good audio editing program (WaveLab LE) and never felt the need to download Audacity, but recently someone mentioned a feature that Audacity had that my program didn't, so I installed it. I checked to see if it could save a file as an mp3 and it couldn't, but when I looked in the help file I discovered the informational statement about downloading the codec from LAME in order to get that functionality. So I installed that codec and found that Audacity, Windows Media Player, and Wavelab can all now save in mp3 format! Why didn't those other two programs tell me to do that? It never made any difference for music since I always play that in random order anyway. I've never been able to tell the difference in sound quality between wma and mp3 even on my home stereo system with a good amp and speakers, so it just didn't matter.
I don't know why the Leaf player can't use the file names or dates for wma files, but it still plays those in random order when it's audiobooks. For music files, in Normal mode it does play in alphabetical order by file name. I don't know why that doesn't work with audiobook wma files. I'm going to post this in the thread "Can't play songs in order" too, since the same discussion has taken place there.