Hi
The noise spectrum is not flat, it is noisy, its power spectral density (which is usually denoted as No/2) is in theory constant over the whole frequencies. the noise generated by matlab is right.
Plus awgn is assumed to be present in all electrical circuits as a product of thermal noise, and this noise can not filtered.
Since you mention a sound card, I assume you are working with audio frequencies, and I guess you filters are low pass ones, if I am not wrong you need to filter high frequency components which are not awgn.
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Sal