What you'd doing is a "noise gate" function.
A few ideas-
Use either the mean volume or 3-6db below the max as the threshold. A more
complicated version would be to examine the level at maybe half-second
intervals and use that to determine the levels of the background and spoken
parts of the clip.
Do the silence removal before the normalize so you're not bringing up the
noise level along with the speech.
Expand the dynamic range (compand) to push the voice level up and the noise
level down. Compand can even do a noise-gate, there's a somewhat cryptic
example in the audio filters doc
(https://ffmpeg.org/ffmpeg-filters.html#toc-compand).
One thing that will bite is if the recording's automatic gain control is too
aggressive and gets the background noise at the start/finish to the same
level as the voice. Not much you can easily do about that but ask for a new
recording.
Later,
z!
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