Using AudioFormat to check the availability of the AAC hardware encoder.

aac_hardware_decoder_detect.m

// Check this: https://developer.apple.com/library/content/qa/qa1663/_index.html
Boolean IsAACHardwareEncoderAvailable(void)
{
    Boolean isAvailable = false;
    OSStatus error;

    // get an array of AudioClassDescriptions for all installed encoders for the given format 
    // the specifier is the format that we are interested in - this is 'aac ' in our case
    UInt32 encoderSpecifier = kAudioFormatMPEG4AAC;
    UInt32 size;

    error = AudioFormatGetPropertyInfo(kAudioFormatProperty_Encoders, sizeof(encoderSpecifier),
                                         &encoderSpecifier, &size);
    if (error) { printf("AudioFormatGetPropertyInfo kAudioFormatProperty_Encoders
                          error %lu %4.4s\n", error, (char*)&error); return false; }

    UInt32 numEncoders = size / sizeof(AudioClassDescription);
    AudioClassDescription encoderDescriptions[numEncoders];

    error = AudioFormatGetProperty(kAudioFormatProperty_Encoders, sizeof(encoderSpecifier),
                                     &encoderSpecifier, &size, encoderDescriptions);
    if (error) { printf("AudioFormatGetProperty kAudioFormatProperty_Encoders error %lu %4.4s\n",
                          error, (char*)&error); return false; }

    for (UInt32 i=0; i < numEncoders; ++i) {
        if (encoderDescriptions[i].mSubType == kAudioFormatMPEG4AAC &&
            encoderDescriptions[i].mManufacturer == kAppleHardwareAudioCodecManufacturer) isAvailable = true;
    }

    return isAvailable;
}