Identify two MIDI controls on your MIDI device.

[controlOne,~] = midiid
[controlTwo,~] = midiid

Move the control you wish to identify; type ^C to abort.
Waiting for control message... done

controlOne =

        1081

Move the control you wish to identify; type ^C to abort.
Waiting for control message... done

controlTwo =

        1082

Create a MIDI controls object that listens to both controls you identified.

controlNumbers = [controlOne,controlTwo];
midicontrolsObject = midicontrols(controlNumbers);

Move your specified MIDI controls and return their values. The values are returned as a vector that corresponds to your control numbers vector, controlNumbers.

tic
while toc < 5
    controlValues = midiread(midicontrolsObject)
end

controlValues =

    0.0397    0.0556

Use midiid to identify the name of your MIDI device and a specified control. Move the MIDI control you want to identify.

[controlNumber, deviceName] = midiid;

Move the control you wish to identify; type ^C to abort.
Waiting for control message... done

Create a MIDI controls object. The value associated with your MIDI controls object cannot be determined until you move the MIDI control. Specify an initial value associated with your MIDI control. The midiread function returns the initial value until the MIDI control is moved.

initialControlValue = 1;
midicontrolsObject = midicontrols(controlNumber,initialControlValue);

Create a dsp.AudioFileReader System object™ with default settings. Create an audioDeviceWriter System object and specify the sample rate.

fileReader = dsp.AudioFileReader('RockDrums-44p1-stereo-11secs.mp3');
deviceWriter = audioDeviceWriter(...
    'SampleRate',fileReader.SampleRate);

In an audio stream loop, read an audio signal frame from the file, apply gain specified by the control on your MIDI device, and then write the frame to your audio output device. By default, the control value returned by midiread is normalized.

while ~isDone(fileReader)
    audioData = step(fileReader);
    
    controlValue = midiread(midicontrolsObject);
    
    gain = controlValue*2;
    audioDataWithGain = audioData*gain;
    
    play(deviceWriter,audioDataWithGain);
end

Close the input file and release your output device.

release(fileReader);
release(deviceWriter);