I have a function for converting a PCM buffer to AAC audio that appears to be correct:

    func convertPCMBufferToAAC(pcmBuffer: AVAudioPCMBuffer, sampleRate: Double, channels: AVAudioChannelCount) -> Data? {
        let audioFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: sampleRate, channels: channels, interleaved: false)!
        
        let settings: [String: Any] = [
            AVFormatIDKey: kAudioFormatMPEG4AAC,
            AVSampleRateKey: sampleRate,
            AVNumberOfChannelsKey: channels,
            AVEncoderBitRateKey: 192000
        ]
        
        guard let outputFormat = AVAudioFormat(settings: settings),
              let converter = AVAudioConverter(from: audioFormat, to: outputFormat) else {
            print("convertPCMBufferToAAC: Failed to create AVAudioConverter")
            return nil
        }
        
        let inputBlock: AVAudioConverterInputBlock = { inNumPackets, outStatus in
            outStatus.pointee = .haveData
            return pcmBuffer
        }
        
        let aacBuffer = AVAudioCompressedBuffer(format: outputFormat, packetCapacity: 512, maximumPacketSize: converter.maximumOutputPacketSize)
        var error: NSError?
        
        let status = converter.convert(to: aacBuffer, error: &error, withInputFrom: inputBlock)
        
        if status == .error || error != nil {
            print("convertPCMBufferToAAC: Conversion error - (error?.localizedDescription ?? "unknown error")")
            return nil
        }
        
        let data = Data(bytes: aacBuffer.data, count: Int(aacBuffer.byteLength))
        print("convertPCMBufferToAAC: Conversion successful, data size: (data.count) bytes")
        return data
    }

convertPCMBufferToAAC: Conversion successful, data size: 86945 bytes
checkOutput: Converted PCM to AAC, size: 86945 bytes

However, converting back to a PCM buffer always results in a frame length of 0:

    func decodeAACDataToPCM(aacData: Data, sampleRate: Double, channels: AVAudioChannelCount) -> AVAudioPCMBuffer? {
        let outputFormat = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: sampleRate, channels: channels, interleaved: false)!
        
        let settings: [String: Any] = [
            AVFormatIDKey: kAudioFormatMPEG4AAC,
            AVSampleRateKey: sampleRate,
            AVNumberOfChannelsKey: channels
        ]
        
        guard let inputFormat = AVAudioFormat(settings: settings),
              let converter = AVAudioConverter(from: inputFormat, to: outputFormat) else {
            print("decodeAACDataToPCM: Failed to create AVAudioConverter")
            return nil
        }
        
        let pcmBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(sampleRate * 2))!
        let aacBuffer = AVAudioCompressedBuffer(format: inputFormat, packetCapacity: 512, maximumPacketSize: aacData.count)
        
        aacData.withUnsafeBytes { (rawBufferPointer: UnsafeRawBufferPointer) in
            guard let rawPointer = rawBufferPointer.baseAddress else { return }
            aacBuffer.data.copyMemory(from: rawPointer, byteCount: aacData.count)
        }
        aacBuffer.byteLength = UInt32(aacData.count)
        
        let inputBlock: AVAudioConverterInputBlock = { inNumPackets, outStatus in
            outStatus.pointee = .haveData
            return aacBuffer
        }
        
        var error: NSError?
        let status = converter.convert(to: pcmBuffer, error: &error, withInputFrom: inputBlock)
        
        if status == .error || error != nil {
            print("decodeAACDataToPCM: Conversion error - (error?.localizedDescription ?? "unknown error")")
            return nil
        }
        
        print("decodeAACDataToPCM: Conversion successful, frame length: (pcmBuffer.frameLength)")
        print("decodeAACDataToPCM: PCM buffer details - format: (pcmBuffer.format), frameLength: (pcmBuffer.frameLength), capacity: (pcmBuffer.frameCapacity)")
        
        return pcmBuffer
    }

checkOutput: Converted AAC back to PCM, frame length: 0
checkOutput: Decoded buffer has zero frame length, skipping playback