.. Getting the violin

Could you switch the violin?

Let's clear couple things first. Get to the lake. After that you can help the musicians with their house. ( After getting them into the house go knock the door, again. )

Switch that old violin with the violin in the store.

He doesn't let you do it. Maybe you could distract him with something.

Like, cause some excessive noise.

There is the toy monkey with cymbals.

Use your telekinetic powers to activate it.

While the shopkeeper is distracted switch the violins.