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I found a way to accomplish this using AVAudioConverter: let audioFileInputBuffer = AVAudioPCMBuffer(pcmFormat: audioFile.processingFormat, frameCapacity: UInt32(audioFile.length))! try? audioFile.read(into: audioFileInputBuffer) let outputFormat = AVAudioFormat(commonFormat: audioFile.fileFormat.commonFormat, sampleRate: audioFile.fileFormat.sampleRate, channels: 1, interleaved: false)! let audioConverter = AVAudioConverter(from: audioFileInputBuffer.format, to: outputFormat)! audioConverter.channelMap = [0] let outputBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: UInt32(audioFile.length))! var error: NSError? let inputBlock: AVAudioConverterInputBlock = { inNumPackets, outStatus in     outStatus.pointee = AVAudioConverterInputStatus.haveData     return audioFileInputBuffer } audioConverter.convert(to: outputBuffer, error: &error, withInputFrom: inputBlock) if let error = error {     print("AUDIO CONVERSION ERROR: \(error.localizedDescription)") } var settings: [String: Any] = [:] settings[AVFormatIDKey] = kAudioFormatLinearPCM settings[AVAudioFileTypeKey] = kAudioFileWAVEType settings[AVSampleRateKey] = outputBuffer.format.sampleRate settings[AVNumberOfChannelsKey] = 1 settings[AVLinearPCMIsFloatKey] = false let outputURL = audioURL.deletingLastPathComponent().appendingPathComponent("mono.wav") let outputAudioFile = try? AVAudioFile(forWriting: outputURL, settings: settings, commonFormat: outputFormat.commonFormat, interleaved: false) try? outputAudioFile?.write(from: outputBuffer) let outputData = (try? Data(contentsOf: outputURL))! But isn't there a simpler way to get the converted mono audio data? Going via a file is slow. I'd like to extract the audio data directly from outputBuffer.