how flat in freq can one go before they reach absolute 0 freq and what would happen to a number in time?

Answer 1

Not sure what your question is. A flat line, Y=constant has no frequency component,.and could be thought of having a zero frequency.

Answer 2

You can't really talk about sound waves reaching zero frequency, because that is simply silent, still air.

But you can certainly create zero frequency electric current. It's called direct current, or DC. Voltages that change over time, usually in a sine wave, are called alternating current or AC.

Circuit analysis in DC is very simple, because you do not have to consider time effects as much. AC is more complex; capacitors change from an open circuit to a closed one as frequency increases. Resonance effects show up. It's a very different kind of process.

Yes, you can reduce a wave to zero hertz and infinite wavelength; but you change it fundamentally in the process.