Speed, tempo and pitch are three different jobs in Audacity. This guide covers each one in Audacity 4 (the clock handles, the pitch and speed dialog, BPM math, reset and render), fixing a tape or record that ran at the wrong speed, slowing a song down to practise, and the Audacity 3 routes.
Noise reduction leaves some sounds alone. How to lower breaths with Clip gain, fix mouth clicks with Click removal and Repair, take out squeaks, coughs and chair noise, and what really helps against room echo in Audacity 4.
Why a pitch change alone does not disguise a voice, how Audacity 4 splits voice effects between the Effect menu and realtime plugins, starting settings for robot, malfunctioning android, demon, FM radio and telephone voices, a free plugin chain you can save, and male-to-female changes.
What ACX’s three numbers measure and how to meet them in Audacity — one chapter in Audacity 4 with a high-pass filter, RMS loudness normalization and a limiter on the Master, a whole book with the Audiobook Mastering macro in Audacity 3, how to fix a noise floor that fails, and why over-processing fails human review.
Copy the broken file, open it in the latest Audacity, then run Audacity Project Tools. What each error message means, what to try when the tools fail, why projects break and how to keep the next one safe.