I dunno how big the sound files are, but mp3 is going to use a lot less storage than uncompressed PCM. Using a smaller flash chip saves money on the BOM.
Bluetooth headphones already need to manage several formats to decode, and the chipset that does that may also decode mp3, and mp3 is easy to work with to prepare the files.
Bluetooth headphones already need to manage several formats to decode, and the chipset that does that may also decode mp3, and mp3 is easy to work with to prepare the files.