Thanks for providing the link. I've checked it out and can see that your store is not actually responsive. If you look at the image below, this is code taken from your site, and you can see that it says 'width ="750"'
This means that in your dashboard you have the width set to 750 pixels, and this will not change. So here's the step-by-step guide to fix this
Go to Music Stores > HTML5 and select your configuration
Tick the 'Responsive' box (you'll see that the width changes from '750' to 100%, as in the screenshot below
You'll know if you've done it right because your code will show width=100%, as in the screenshot below