In the newest version of the app on my iPhone 6, I have all sounds and music turned off. The app is normally completely silent. However, when I open a pack of cards, the sound effects play at full blast anyway.
Same with me. And I have Android 5.1
It has always been thus. 
Completely infuriating. Wishing this would have been patched by now. Scares me every time.
This is unfortunately still an issue, please patch this!