You keep saying "breath sensor", but it really is a "noise of blowing sensor". It isn't detecting the moisture in your breath. Just making sure you know that.
As I said, simply removing a piezo from a case may actually make it less sensitive at picking up sound. There is a reason that microphones and speakers come in cases, to separate the front from the back to avoid having the sound partially cancel is the primary reason.
Seriously, just add an amplifier. A simple Op Amp circuit, or an LM386 audio amplifier.