It's safe bet that every keyboardist has used a "ROMpler"-type instrument. These use brief audio samples of real or electronic instruments or effects. Unlike a classic sampling instrument where users could record their own custom samples, a ROMpler contains a fixed set of raw samples to create sounds ("ROM" is an acronym for "read-only memory," i.e., permanently programmed).
In a sense Harmonia is a gussied-up ROMpler instrument in that it also uses a fixed collection of audio samples to create sounds. The difference is that Harmonia lets you do a heck of a lot more with those samples in order to create insane (or slightly-saner) evolving tones, but because its raw sample library was custom made to be quirky and unique, there are plenty of great sounds to be made without using any of Harmonia's fancy modulation capabilities. Here's how:
Click Harmonic Level Bar 1 at the bottom and drag it vertically to its max setting. (Harmonic Level Bars can actually be clicked and moved anywhere in their vertical travel. They're also fabulous places for harmonics to meet other lonely harmonics and... oh, nevermind.)
Set the Amp Envelope Attack and Release controls to desired length and we've got a lovely faux-Mellotron.
By selecting various raw waveforms, Harmonia can be used just like any other basic ROMpler instrument. You can also fatten up sounds by duplicating Oscillator 1's settings with Oscillator 2 and altering the Detune knob a bit for natural chorusing (the Utility button Copy Oscillator command does a swell job of this).
Of course, you might want to experiment with raising additional Harmonic Level Bars to add additional timbral interest. This segues us right into the next section...