Looks like it is possible to play music using the HD Rumble in the JoyCons. By modifying the frequency of the samples in the bnvib files I managed to make the starting part of the Mario Theme to play (sort of). I wonder if in the future a homebrew app will be made to allow playing midi files through a Joycon, but I doubt it as it is very limited on what sounds you can do
File download: https://mega.nz/#!jQFT2S7J!HAllR3k9siuGO4GX0PJ5-mhSyaR3qzE4G1Awh3a7yrQ