hi,

For piezo sounder you should use parallel connected coil. Both capacity of piezo sounder and inductivity of the coil make resonance and increase voltage over the net.
Indeed, you should not connect such net to a pin directly but use a transistor (+protect diode probably) instead.
Unfortunately, most piezo sounders have resonance frequence between 2...8kHz and so "variable frequency tone" is limited.

Better variable of tones may be done with standard low-resistence speaker (for example those with 4...16 ohm). Here you should add pre-gain schematic. For example, I use transistor TIP125 + big enough capacitor.

Regards,
Oleg