In Epiode 378 of One Piece, what is the first song that Brook played?
The song is called Minato Mura.

After some additional searching I found that the piano solo is part of the song called Village Harbour (or Minato Mura in Japanese), which is supposed to be part of the second song of the second OST collection as shown on Youtube



More information:


Youtube  
Piano sheet on Everyonepiano