Lemme see if I understand your questions correctly...
The percentage of sound coming from the muzzle is simply calculated. If a completely unsuppressed firearm has 150 decibels, but the same suppressed firearm has 120 decibels, 30 of which is converted to heat energy via the silencer, then it 20% of the energy from the muzzle blast has been converted to heat energy.
You asked for the percentage of energy conversion. This is different from calculating the percentage of reduction in noise. Decibel ratings are logarithmic, not linear. This means that 200 decibels is 1000 times louder than 100 decibels; i.e., 200 decibels is NOT 100% louder than 100 decibels.
There are five sources of noise when operating a gun: action noise, bullet impact noise, muzzle blast, sonic signature, operator noise (wiki says there are three, but I've always been taught there are the five mentioned).
With a silencer, if I understand your question correctly, you are also asking if there can be noise directly emanating from the body of the suppressor. I would hypothesize yes, there can be noise emanating from the body of the suppressor. So, that would be a sixth source of noise, however minimal the noise. To clarify, you aren't talking about the nose from the front/exhaust/muzzle of the suppressor, rather, you're talking about from the suppressor body itself. If you want to get really technical, then the noise source would come from the expanding gas hitting the walls/cylinder and baffles, thereby creating noise. The analogy which I would state is that the gas is like a baseball bat, and the baffles and cylinder are like a plank of wood. When the baseball bat hits the plank of wood, there is sound created. Similarly, when the expanding gas hits the cylinder and baffles, there is also sound created.
How much is this noise? I can't imagine a way to manually or directly measure this, as the sound would be too minimal, compared to the muzzle blast, to manually or directly measure. You would have to extrapolate such data based on various known variables; variables which I don't have.
I hope someone can extrapolate the answer for us.