Im currently working on VAD (voice activity detection) topic. Since I'm beginner in digital signal processing, I've started to wonder if calculating single frame energy (after aplying FFT to it) and looking for values around 4Hz and bandwith (for example) from 200Hz to 600Hz would be as effective as checking for 4Hz modulation energy after finding MFCC coefficients?
From my understanding of your question, you want to ask whether one should look for energy(of FFT coefficients) of bands directly, or one should use MFCC's (energy) for making VAD decisions.
As MFCC based approach is to mimic the human auditory systems response, it will be better to use MFCC coefficients.
if one uses same bands as MEL Filterbank for calculating band-wise energy (after taking FFT), than the results will be similar to using MFCC'S, as MFCC's are just the transformation from FFT coefficients.