Songist is a free application that works both on ChromeOS & Google Chrome browser and allows you to play songs stored in your USB stick. Using this app you can play music offline using a simple USB drive on your Chromebook. Other chrome users may also use it for playing music stored on their hard disk drive.
Add Songist app in you Chrome, launch it and then you can scan for music stored on your USB or local drive. After scanning it automatically classifies all your music into genres, artists, albums, tracks and queue. The application works perfectly without any internet connection and works as an excellent offline music player for playing music you already have.