How to chose a kernel function for a FIR low pass filter in audio processing? As the answer of a previous question, I was suggested to use a FIR filter to act as a low-pass filter to reconstruct the envelope of a sound wave (edited, emphasis mine):


  As I said in a comment, you can get the envelope of a signal by
  running it through a lowpass filter
  
  [...]
  
  
  Implement a lowpass filter (FIR) by creating a filter kernel of appropriate length M (h(M) ). Note that your FIR filter
  (sin(x)/x) should normally be multiplied by a window function (e.g
  Hamming, Blackman, etc).
  Convolve your signal with the filter kernel. 
  
  
  [...]


I'm having a hard time choosing/finding a proper filter kernel function. The quote above suggests sin(x)/x. Is this a standard kernel function? Does it have some magic properties that make it particularly suitable? What should I check if I consider another kernel function?
Yes the sinc kernel $$h[n] = \frac{\sin(\omega_c n)}{\pi n}$$ is the theoretical result for the ideal lowpass brickwall filter whose gain is one and cutoff frequency is $\omega_c$ radian per samples. 

Your filter impulse response will be samples of that function. Truncate it symmetrically to some finite length, and then apply a window on those samples. Windowing will help you achieve a much smoother response.