>>13501101you see that the spectrum is mirrored (symmetric) this is fft effect
also from the spectrum it is obvious that the signal is not really distorted, amplitudes are the same, its just noise added to it in a certain range of frequency
so in this case you do not need to use a complex FIR or IIR filter to remove distortions
a simple lowpass filter will do, make the cutoff frequency at 10 kHz
order 1 of the filter will be sufficient but since its a digital filter you can easy use higher orders (wont really have any effects, except slew rate will be higher so, attenuation of frequency > 10kHz will be higher)
matlab has functions for lowpass FIR filters
for audio most information is in frequency below 2 kHz anyways, so filtering any higher than that wont have big information loss
