Those are the only others I see in the source code, and they're documented in Inside Playdate with C, but I'll have to ask why they aren't exposed in the Lua API. Most likely an accidental omission. Thanks for bringing it to our attention.
Thank you! Checking the C documentation haven't even crossed my mind. Though it doesn't seem like the variables playdate.sound.kFormatADPCMMono or playdate.sound.kFormatADPCMStereo exist. Maybe they're not exposed in the Lua SDK?