EQ (equalization)
Turning specific frequency ranges up or down: cut the mud at 250 Hz, add air up top, and a voice suddenly sounds expensive.
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What it is
EQ is volume control per frequency range instead of for the whole sound. The shapes you get are a high-pass (roll off everything below a corner frequency, as fast or slow as the slope decides), a shelf (lift or drop everything above or below a point), a bell (a hump or dip centered on one frequency, with a width set by Q), and a notch (a surgical slot for a single hum or whistle).
The map for voices, roughly: 80 to 100 Hz and below is rumble, traffic, and desk thumps, so high-pass it away. 200 to 400 Hz is 'mud'. 300 to 600 Hz is 'boxy'. 3 to 5 kHz is presence and intelligibility. 6 to 8 kHz is where sibilance bites. Above 10 kHz is 'air', that expensive broadcast sheen. Sweep a narrow boost around until the ugly thing gets uglier, then cut there instead.
Gotcha: cutting beats boosting. A boost also lifts the hiss and room noise living in that band, and boosts make things louder, which your brain hears as 'better' regardless of whether it is. Match levels before and after or you will EQ yourself into a corner.
Ask AI for it
EQ this voiceover for a clean, broadcast-style tone: high-pass at 80 Hz with a 12 dB/octave slope to remove rumble, cut about 3 dB around 250 Hz with a medium Q to clear the mud, cut about 2 dB around 400 Hz if it sounds boxy, add a gentle 2 dB presence lift around 4 kHz for intelligibility, and add a soft high shelf of 1 to 2 dB above 10 kHz for air. Keep the moves subtle, and de-ess afterwards if the presence lift makes the S sounds sharp.