Stretching audio in Swift

ScaleAudio.swift

// from https://nonstrict.eu/blog/2023/stretching-an-audio-file-using-swift/

import Foundation
import AVFoundation

func scaleAudio(inputURL: URL, toDuration targetDuration: CMTime, outputURL: URL) async throws {
    // Load info from the input audio file
    let inputAudioAsset = AVAsset(url: inputURL)
    let inputAudioDuration = await inputAudioAsset.load(.duration)
    let inputAudioTimeRange = CMTimeRange(start: .zero, duration: inputAudioDuration)
    let inputAudioTracks = await inputAudioAsset.loadTracks(withMediaType: .audio)
    guard let inputAudioTrack = inputAudioTracks.first else {
        fatalError("No audio track in input file.")
    }

    // Create a composition with the current audio added to it on a track
    let composition = AVMutableComposition()
    guard let audioTrack = composition.addMutableTrack(withMediaType: .audio, preferredTrackID: kCMPersistentTrackID_Invalid) else {
        fatalError("Failed to add mutable audio track.")
    }
    try audioTrack.insertTimeRange(inputAudioTimeRange, of: inputAudioTrack, at: .zero)
    
    // Scale the whole composition to the target duration, this stretches the audio
    composition.scaleTimeRange(inputAudioTimeRange, toDuration: targetDuration)

    // Setup an export session that will write the composition to the given ouput URL
    let exportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A)
    exportSession?.outputURL = outputURL
    exportSession?.outputFileType = .m4a

    // Do the actual export and check for completion
    await exportSession?.export()
    guard exportSession?.status == .completed else {
        fatalError("Export failed, check `exportSession.error` for details.")
    }
}