Why not just build one in? I guess I'm accustomed to assault rifles having flash suppressors because the one that I've actually handled and lugged around and shot with, the Valmet RK 62, does have a suppressor built in, a distinctive feature [1].
I'm not seeing anything about a built in sound suppressor just a flash suppressor which is a fairly common feature on modern assault rifles and their civilian variants. You don't generally build in a suppressor because they're technically wear items as the hot gases will slowly erode the baffles over time. That plus their extra weight and cost meant most militaries only issued them to units that were built for stealth missions in the past.
A bit yeah though they went from 20-inch M16s at the beginning of the GWOT to 14.5 or less as the main weapon so there was space to slap a suppressor on but you'd give up the maneuverability gains of losing that length. The NGSW winner is pretty short barreled before the suppressor is added and it required particularly spicy (high pressure) ammo to still be effective at that short of a length.
[1] https://en.wikipedia.org/wiki/RK_62