I'll mirror pert's second recommendation..
but say perhaps try to prototype/develop using a Pro-Micro instead..
Can be used as a HID device/emulator... and you dont need to mess USB/serial stuff until you are happy with what you got.. and need to port over to the required MEGA..
This equates to the same approach as having a 'keyboard' marco or something..
Questions:
Open? With what? Windows Media Player? what application on your 'computer' (PC? MAC?) will be doing the real 'playback' of the audio clip?