I have a feeling it might be better in startup.sh, make a backup copy first though.
sudo nano /opt/musicbox/startup.sh
place it at the end after the autoplay setup bit, or amend the autoplay setup bit, have a look here for some help with that.
It’s a while since I used cron, I think it fails it’s because it tries to play it before musicbox is ready, then gives up, the alternative is to ask it to sleep for a while, however I didn’t find that very successful.