Audio Filtering
Most built-in filters immediately support filtering of the audio signals.
In[1]:=

a = ExampleData[{"Audio", "Apollo11SmallStep"}, "Audio"]
Lowpass filter.
In[2]:=

lp = LowpassFilter[a, Quantity[800, "Hz"], 101]
Highpass filter.
In[3]:=

hp = HighpassFilter[a, Quantity[1600, "Hz"], 101]
In[4]:=

Periodogram[{a, lp, hp}, 2000, PlotRange -> {{0, 3000}, Automatic},
Frame -> True, ImageSize -> Medium,
PlotLegends -> {"Original", "Lowpass", "Highpass"}]
Out[4]=

Remove noise using a Wiener filter.
In[5]:=

WienerFilter[a, 25]