A directory with the name 01 must be created on the SD card. This has the advantage that you can create several folders (02,03....) and then simply name them 01 if necessary.
The six sound files are then placed in this folder, whereby 001.mp3 corresponds to the first button and 016.mp3 to the last button.
Sound file 020.mp3 contains the text "Battery low". I created this with Text to Speech.
030.mp3 and 031.mp3 are played randomly after a few minutes to remind you to switch off the device when it is not in use (creepy but effective).
Battery Low example:
Power Off Example: