DFPlayer -- how to use subfolders 01 02 etc.

Okay, I have the MP3 player working in a simple circuit (TX/RX, speaker). It plays files out of the /MP3 folder on the microSD.

The documentation suggests that /MP3 will support up to 256 files (one byte of file number) and that additional files can be placed in folders 01 02 etc.

I've tried using /MP3/01 and /MP3/02 and selecting files using "mp3_play((folder<<8)+file)". This correctly places the two bytes in the command: "7E FF 6 12 0 2 1 FE E8 EF" for folder 2, file 1. The file did not play.

I've tried sending a "select folder" before "play":
7E FF 6 F 0 0 2 FE EA EF
<delay 10ms>
7E FF 6 12 0 0 1 FE E8 EF
I've tried using the folder/file combo (folder high byte, file low byte) as specified in the DFPlayer Mini Manual: "7E FF 6 F 0 2 1 FE E9 EF" for folder 2, file 1.

I've also tried putting the /01 and /02 folders at the root of the SD instead of in /MP3.

All of these were generated using the Arduino DFPlayer_Mini_Mp3 library, pulled from GitHub on 10/5/2016. Arduino IDE 1.6.11. I *did* create a mp3_set_folder(int f) function based on mp3_play(int f). The strings are pulled by dumping send_buf after the command was processed.

Nothing works.

Does anyone have any suggestions or working code that uses subfolders?

Thanks!