Don't use a piezo disc or piezo buzzer unless you want
an audible *tone* i.e. something that's good at buzzing /
beeping at a given relatively fixed frequency.
If you want to reproduce a wide(r) range of audio frequencies through it as a speaker, you should get something that's sold as a speaker.
Check digikey.com, mouser.com, newark.com,
amasci.com,
http://www.goldmine-elec.com/
et. al.
You'll have a hard time finding something that's BOTH
the diameter of a quarter or so AND as flat as a quarter
that's good at being loud and acts as a good speaker
over the audio range. For full fledged speaker function,
look for something the thickness of 3 to 8 stacked quarters,
or something a bit bigger in diameter but thinner.
Check your local junk cell phone or small earphones or
headphones for cheap or free tiny speakers, though
the cellphone / headphone ones will tend to be a bit
thicker, and the earphone ones don't put out so much
audio power.