From CCRMA Wiki
The following code shows how to run a low-pass filter in matlab. You'll want to change 4000 to your desired cutoff.
[x, fs] = wavread(file); cutoff = 4000/(fs / 2); % cutoff 4 kHz [B, A] = butter(10,fCutoff,'low'); % construct low pass filter. 10 should make it nice and sharp. x = filtfilt(B,A,x); % do the filtering and clobber x with the result. a = miraudio(x, fs); % construct a miraudio object and go on our merry way.