MATLAB: Attempting to record and play simultaneously using DSP Toolbox shows errors. Also, a way to vary volume of dsp.AudioPlayer

dsp.audioplayerdsp.audiorecorderdsp.toolboxsimultaneous record and playvary dsp.audioplayer volume

I want to record and play a song at the same time. From what I know, you can do so using DSP or DAC toolbox from Matlab. Since I had no DAC, I'll go with DSP instead. Also, I'm using Mac and Matlab R2014b. Mac has a CoreAudio sound device, which as far as I know can do full-duplex operations, so I'll be using that.
Anyway… I have the ff code:
In = dsp.AudioRecorder('SampleRate',Fs,'DeviceName','Built-in Microph',...
        'NumChannels',1,'SamplesPerFrame',60000);
Out = dsp.AudioPlayer(fs,'DeviceName','Built-in Output');
step(Out,monod);
audio = step(In);
Fs is the recorder sampling freq, while fs is the song sampling freq I obtained by using ' audioread'. However, firstly, it did not record and play simultaneously. Moreover, running the section again displayed the following error:
Error using AudioPlayer/step.
A given audio device may only be opened once.
Does that mean step is incapable of replays? Was my assumption correct that this should do a simultaneous recording and playing using DSP Toolbox (and that I just need to obtain a sound card)? Also, is there a way to vary the volume of DSP version of audioplayer (i.e. dsp.AudioPlayer) like the usual audioplayer? I'd really be grateful for some guidance. Thanks!
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