For whatever reason my new iPhone 6s doesn't always play system sounds, such as the Siri chime (there's also no Taptic feedback), the "magnetic click" when I lock my iPhone (which sometimes does work), and others. Even though some other sounds do play, such as the charging chime.
When I had my iPhone 5s running iOS 9, though the Siri sound didn't play, I got vibration feedback when I summoned Siri. My iPad Air (also running iOS 9) does play all of the sounds.
So is this an iPhone 6s specific thing, where Apple's trying to limit sounds to make a more Apple Watch-like experience, a software bug, a Setting that I missed, or something else entirely?