Note how the old is flattened on the strike-point from years of hitting the steel strings. This hammer cannot have good tone in this condition.
Voicing is a bit like the equalization on your stereo, where low, middle and high frequencies are blended.
The attack of the piano can be altered as well ( a key pressed slowly with more weight should have a different tone character than when pressed quickly with less weight).
Here, the technician begins to shape tone for what it could be, by altering the hardness and density of the hammers. The “singing” piano tone is because of the voicing of the hammers as much as it is the player. The acoustical qualities of the room, musical demands and player preference are taken into account.
I think of the piano as an orchestra, with a big resonant bass, working through the middle range to the high strings and piccolo in the treble with as much gradation in tone colour as possible. It should be able to produce a whisper or a blare, without merely being a pingy sound that`s more or less loud. It should be resilient.
Voicing is best done after tuning and regulation, when the piano can really be evaluated.
Getting a piano to do its best is a work in progress. It costs money. How serious are you about protecting your investment and enjoying your piano? You wouldn’t buy a car and only put gas into it, would you?
A properly voiced piano is a revelation to pianists of all levels.