Plays and records audio, including ACC and MP3 files.
To put music on there you can either just chuck it in the root of the SD card, or make a folder/s there. When listing music in the software it will first show you all folders together that contain music, even if they're subfolders. So don't bother organising stuff by putting artists in their own folders, as a folder without music files in it won't appear in the listing.
According to Nintendo's website the following formats are supported:
So pretty standard, but no OGG >:-|
You can also make a small playlist of songs that will be used for StreetPass. I only found out about this is March 2020, so not sure what exactly gets shared, but it looks like it makes a 'top 10' list of songs based on that, and makes a note when your tastes match with other people's.
※ Exporting files for the 3DS on the Foobar2000 page.
There're lots of fun features when playing back music!
On Linux I use mkfs to format the SD card. Guides say you need to format with a block size of 32kB/32768B, but mine is 4kB/4096B and seems to work fine? Does seem a little slower than the card that came with the 3DS? Haven't had any luck with the recommended block size. Very likely I just don't understand disks stuff enough.
# mkfs.fat [disk]
Moving stuff to a new card is easy, just copy the directories over.