Pitch Shifter

Two stages chained. First, ffmpeg lies about the sample rate. Playing the file 'faster' shifts pitch up an octave for each doubling of rate. Second, atempo time-stretches back to the original duration. The pitch shift survives; the duration cancels out. Result: pitch changes, tempo stays put.

Up to about ±5 semitones, yes. Beyond that the formants (the resonances that make vowels sound like specific vowels) shift along with pitch, which is why aggressive up-shifts sound chipmunk-y and aggressive down-shifts sound muddy. Tools that preserve formants separately (Auto-Tune, Melodyne) sound more natural at extreme shifts but cost money. For modest shifts this is fine.

Fine-tuning between semitones. 100 cents = 1 semitone. So +50 cents is a quarter-tone up. Useful when a live recording is slightly sharp, when you're matching a sample to an out-of-tune piano, or when you want a deliberate detune effect on a doubler.

Roughly. 'Key change' is the musical term. Moving a song from C major to D major is a 2-semitone pitch shift. The technical operation is identical. Songwriters use this to transpose a song into a singer's comfortable range; producers use it for sample matching.