Hello,
I am currently creating TonUINO audio messages in French (and updating the text file, which I will pull request soon). And I got curious about the advert and mp3 folders. Here are my findings, in case that would be useful for others.
According to the DFPlayer documentation, sound files under advert are meant to be played while allowing to resume music playback afterwards. The mp3 folder just seems to be a special folder, accessible with dedicated functions (see sample code), therefore quite useful for audio interface messages.
Correct?