Volume Modulation

…Plugin for specific hertz?

Please try using a full sentence, then we may understand what you are asking.

What is, “volume modulators available”?

The “Variable Tremolo” effect is a volume modulation effect: http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Variable_Tremolo

What is the difference between tremolo frequency and tremolo amount?

Is the amount at 100% an unmodified, modified signal?

It tells you on that page doesn’t it?