No, you can’t tell the Silence Generator what to generate. It generates zero amplitude samples (absolute silence).
Long method: assuming your room tone track is the length of your speech track, do the following using UP/DOWN arrow to change the track focus and ENTER to toggle the focused track’s selectedness:
- Select the breath you want to replace.
- ENTER to deselect the breath.
- UP arrow on your computer keyboard.
- ENTER to make the same selection in your room tone track.
- COMMAND + C to copy
- ENTER to deselect the room tone.
- DOWN arrow.
- ENTER to select in the speech track.
- COMMAND + V to paste in the speech track.
Shorter method: Use the Punch Copy/Paste Nyquist plugin. You only need to copy the room tone once, then paste using the “Looping” method so that the paste always fills the selection rather than shortening or lengthening it. See here for how to install Nyquist plugins: Missing features - Audacity Support.
Gale