The order of file names 0001 0002 is not the order of file names,

[semibank]

The order of file names 0001 0002 is not the order of file names, but the numbers are played according to the file modification date.

this library does not read files in order of name, but rather determines the order based on the file modification date. Please correct the error

The way to check for errors is to set the most recent file modification date to 0001xx.mp3.

If you designate a file with an old modified date as 0002xx.mp3 and play 0001xx on Arduino, you can see that 0002xx is actually played.

I don't know why the number is determined by being rearranged in order of modified date.

[zoldaten]

the library DOES NOT play in order files!
even i set myMP3.play(3) - it plays not 003.mp3 but some other file.

spent hours while found this issue!

[zoldaten]

i found the way out.
i create folder on sd card named '01' and put there all my files:

then in .ino file i called:
myMP3.playLargeFolder(1,35) , where i is track number.
and it starts working as expected.

[prodestrian]

This is a hardware limitation of the DFPlayer itself, it's a known issue and not something this library has any control over (it's the same with all DFPlayer libraries). It's actually nothing to do with the file modification date, it's the order in which the file is written to the SD card.

If you copy 0002.mp3 then 0001.mp3 to the SD card in that order, then playFile(1) will play 0002.mp3. This is even documented on the official website:
https://wiki.dfrobot.com/dfplayer_mini_sku_dfr0299#Copy%20your%20mp3%20into%20you%20micro%20SD%20card

NOTE: The order you copy the mp3 into micro SD card will affect the order mp3 played , which means play(1) function will play the first mp3 copied into micro SD card.

This is only a problem with files in the root folder though, as @zoldaten pointed out above you can work around this by creating folders. Even one single folder should be fine.