`_segmented_music` mixed the video's audio with the shaped music via
`amix=inputs=2`, whose default `normalize=1` scales every input by 1/inputs
(x0.5, -6 dB). Unlike `_mix` and `_full_mix`, this path has no `loudnorm` stage
afterward to re-normalize, so the narration was permanently attenuated across
the entire timeline — including the stretches where the music volume expression
evaluates to 0. A one-second music segment quietly dropped the narration by
~6 dB for the whole video.
Add `normalize=0` to the amix: the music is already scaled to `music_volume`
by the `volume` expression, so speech passes at unity. Verified with ffmpeg —
narration in a no-music region tracks the stereo/aac conversion baseline
instead of sitting 6 dB below it.
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