I've never seen any speaker that had to have one pin grounded. Any transducer can be driven with ground as a return connection, though... including a piezo disc. All the speakers I've ever seen have two pins. So do piezo discs (well, 2 wires instead of pins usually).
Also, is it possible to control a piezo disc via potentiometer?
This is not necessarily the case in a bridge configuration both speakers are "active" is neither is connected to the amplifier common (aka ground).
Your piezo driver may well have one lead connected to common.
From a component standpoint both have two leads that must be driven such that there is current passed between them.
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Well, that turns out to be nonsense, but it does make me wonder from where you got that idea?
The point is that piezo devices have a really high impedance whereas electrodynamic speakers generally have a quite low impedance, so they have somewhat different use cases.
Some piezo discs have a third connection to facilitate using a single transistor as an oscillator to drive them. Otherwise piezo speakers electrodynamic speakers both connect either to a single-ended drive (one side grounded) or a "bridge".
I think you misheard me. electromagnetic speakers are connect to one pin to the board and one to ground. Piezo ones are connect through two pins and no ground.
No,we did not mishear you.
We want to know from where you obtained this most peculiar (and clearly erroneous) suggestion? 
from another forum
One thing to consider for full Arduboy sound compatibility if adding an amplifier:
The Arduboy’s piezo speaker is connected across two pins, not to one pin and ground. This creates a simple two channel mixer. If you play something on one pin and something different on the other pin, the two outputs will be combined and played on the single speaker. Or, if you play something on one pin and the inverted version of the same thing on the other pin, you will get double the voltage on the speaker and thus will get higher volume. (You can play sound from just a single pin by holding the other pin low.)
To use those same two outputs with a single channel amplifier, you would need a mixer circuit on the input. Possibly, this could be just a simple resistive divider.
However, another “trick” that the Arduboy uses is to set the pins to inputs as a way of muting the sound. Normally, both pins are set as inputs for this but it would still work to mute sound playing on one pin by only setting the other pin as an input.
The case where both pins are set as inputs would probably still work with some mixer circuits but handling muting of sound on one pin by setting the other to input (thus high impedance) could be difficult to accomplish with a mixer/amplifier.
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Arduboy is not a speaker. It may contain a speaker but it is not a speaker.
A speaker has two input wires....period.
And, there is no such thing as ground in a typical circuit. It is really the common connection to whatever circuit you have.
The Arduboy’s piezo speaker is connected across two pins, not to one pin and ground. This creates a simple two channel mixer.
This would be a simple bridge configuration. old old concept. Many high power systems use a bridge configuration with ...... wait for it....... a two wire speaker.
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Most music is frequencies half that or less.
If you take the cone off a coil drive speaker, it would have such a response.
So give the piezo a cone to shake, stiff plastic cup works so-so, done that much.
Mono speaker cable has 2 strands, signal and ground, there has to be a current return.
The voltage difference between signal and ground makes the volume.
Arduino sound is digital. It's all between ground and VCC over ON/OFF intervals of 2 ms or less. That works better at CD quality that an ARM chip Arduino like the Due can do.
This thread is seriously weird.
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Well, we have the OP making an uneducated assertion and when questioned as to why, replies
Clearly does not comprehend our disdain - and the reasons - for miscellaneous/ unspecified other "internet forums". 
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As for making them louder, try experimenting with different surfaces.
For buzzers with a small hole, you can get a pretty good boost by finding a short length of plastic tube, conduit for electricians or overflow pipe for plumbers and fix that over the hole.
Makes a big difference.
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Will it be loud if the disc is on perfboard? (pcb board with holes)
Solder one down and test it, but I suspect you would only notice a little difference at most.
You aren't providing much feedback about what you've tried...
Alrught so i got my piezo disc and how do I test them?