Its can be done using root mean square (RMS).

One way to build your own rms function using python is:

def rms( data ):
    count = len(data)/2
    format = "%dh"%(count)
    shorts = struct.unpack( format, data )
    sum_squares = 0.0
    for sample in shorts:
        n = sample * (1.0/32768)
        sum_squares += n*n
    return math.sqrt( sum_squares / count )

Another choice is use audioop to find rms:

data = stream.read(CHUNK)
rms = audioop.rms(data,2)

Now if do you want you can convert rms to decibel scale decibel = 20 * log10(rms)