Sound insulation tends to hold in heat as well, so now you need the weight of extra cooling as well.
You need to think like an engineer. You can't add things in isolation. Every one thing you change has effects that cascade and change a dozen other things. It could be that all the necessary changes would make it too heavy.. Or make it too large... Or make it use too much energy... Or the expense of everything added and the studies needed to recertify it were prohibitive.
I don't sound like an engineer, Mr anonymous coward troll, FYI, PhD in Engineering Physics from Purdue right here, and I've been wrenching on engines since I was 5.
Its not a hard problem to make things quiet. Boston Dynamics solved the DAMNED HARD problem of walking, stability and self-guidance. Compared to these problems, adding a bit of quiet is trivial. Its clear from the videos that these are prototype mules, final versions would have had paneling, and would have used probably a more specialized engine. My guess is they just grabbed some farm equipment engine off the shelf to get a prototype together and didn't pay much attention to noise at this early stage, noise is an easy problem to deal at a later stage. Done right, a better muffler, some sound deadening panels would maybe add 20 lbs at most, and you construct the panels such that the dampen noise, but still provide path for heat exchange. Dealing with noise is a very standard and well worked out engineering problem, and its nothing compared to the problems that Boston Dynamics has overcome.
The IQ of the group is the lowest IQ of a member of the group divided by the number of people in the group.