I've used both models extensively too, and while having them both in hand called my voicemail (Google Voice), left a voicemail with each, and then played back on my mac with quality speakers. The 5C is noticeably less clear and more muffled, always, compared to the 5. The 5s has a better mic than both of them, but only marginally better than the 5. The 6 is about the same as the 5s. I haven't had a chance to test a 6plus. The Samsung Galaxy 6 mic BLOWS AWAY the iPhone 6 mic in clarity and definition, which surprised me, as Samsung Galaxy 1-4 had horrible microphones. I guess they decided the spend the extra $8 and put a semi-quality mic in. The Apple mic is at best a $2 item.