Recording function broke down with iPhone 6s

I got a new iPhone 6s iOS 9.2.1 and the recording quality turned very very poor. I used to be totally satisfied with the sound when I used this app on iPhone 5c. Now with iPhone 6s, it sounds heavily distorted or kind of doubled.
Also it might be helpful to note that when I plug an earphone in to the phone (no need to use a microphone, just an earphone will do), this problem ceased to occur.

既に書かれているように、iPhoneを6sに変えたところ録音が歪むようになってしまいました。以前は5cを使っており、問題ありませんでした。応急処置ですが、イヤホンを挿した状態で録音すると従来通りきれいに録れるようです。

XY musician about iRig Recorder LE, v2.0.1