To address your comments/questions Quetscher–When playing the blues, I do not worry about key click too much. I welcome key click as part of the sound (to dirty it up and add the percussive effect as you mentioned). If I don’t want the key click when playing a soft or smooth passage, I just hit the keys softer to remove the key click. Actually, it kind of happens organically: one tends to play aggressive passages hard, adding key click. One tends to play sweet passages softly and gently without adding key click. Sometimes I just bang on the keys without blowing into the melodica to add percussive effects to the music. The only time key click does not sound good to me is when excessive high frequency EQ is added to the signal chain, something I normally do not do. For music other than blues, I tend to minimize the key click through my playing technique.
I agree, singing and humming into a melodica provides a wide range of effects depending how you do it. I learned about the possibilities a year ago, and have toyed with it since then. But it is very difficult to do, and there is much to explore, so I only work on it a bit at a time. It will definitely be a technique that I add to my arsenal for special effect.
My descriptions of the tracks were not very clear. In terms of distortion, I think it can be achieved several ways based on my experience with the Hohner Piano 26/27s. (Other melodicas may NOT respond the same way.) With a Hohner piano 26/27 (HM-26, HM-27) you may be able to get distortion by:
1. Cranking up a tube amp
2. Using distortion pedals
3. Playing hard
4. Playing chords (all the harmonics from multiple notes mixing together in the body of the melodica offers a distortion)
5. Playing dissonant intervals or dissonant chords (accentuates the harmonic distortions as noted in item 4)
6. Pressuring the sound chamber by holding the Hohner Piano 26/27 tightly against your chest while playing
7. Getting other reeds to respond in sympathy with the note you are playing by pressing most of the melodica against your chest, but leaving some of the sound holes exposed (too complicated to explain here in detail, and still under investigation).
8. Humming and singing into the melodica as you play notes
So rather than try to explain my poorly worded descriptions of the tracks, I would say that any distortion you hear was created by one of the above techniques.