There are several depictions of musical instruments resembling pan flutes on bronze situlae from middle Europe like Vace and Certosa, showing their usage during festivities.

Actual musical scores that survived the ages... I don't know, but I highly doubt that. As you said, Celts had no to only very little written record and beside their rich archaeological heritage they left us to discover most of what we know about this Iron Age culture comes from Romans and Greeks and they liked to describe them as more barbaric as they (probably) were in reality.