Siri Knows When to Shut Up
There are times you want Siri to talk to you and times when you'd rather just see an answer or reply on your phone.
In the iOS 9 settings you can tell Siri to skip audible feedback when the iPhone is on silent mode. This optional setting allows you to use Siri in more locations where you are fine talking, but you don't want Siri to read an answer aloud.
When connected to headphones, Bluetooth or a CarPlay system Siri will revert to audible answers and feedback automatically, even in silent mode.