I've just tried the Modulino Buzzer for the first time with the Simple melody sketch and it's very faint, presumably because it's a piezo at 3.3 V. What is the optimum frequency?
Most small piezo elements have a resonance peak roughly in the 2 kHz to 4 kHz range, with many centered around 2.7–3.2 kHz. That’s where you’ll get maximum sound pressure for a given voltage. If you are playing a melody, transpose it up by one or two octaves. For example, if you're playing C4, play C6 or C7 instead .
There isn't much information available on the data sheet but the SPL (sound pressure level) spec is 4 kHz @ 75dB. That's slightly louder than a normal conversational voice.
Even at the optimum it's still not going to be loud.
Do you need something louder?
Hi, @leon_heller
Can you please post your code and a picture of your project?
So we can see your component layout.
Tom.... 
I tried 4 kHz. That gave reasonable output.