Moving average filter for sound in MATLAB
1.) The moving average filter works as a low pass filter. Consequently, it will attenuate the high-frequency part of the signal and passes the low-frequency part of the signal. Therefore, the output signal should sound flat. 

2.) If we change b0= 0.5 and b1 = -0.5, the filter works basically as a differentiator, subtracting one sample from another sample. In this case, the filter works as a high pass filter, attenuating the low-frequency part of the signal. Therefore, when listening to the output signal, the high-frequency part of the input signal should appear amplified.