When going to stealth mode, AI should check their inventory for a compatible silencer and equip it if they are able to. Right now, they stick to going loud.
Alternatively, there should be an option in the unit commands to "go loud" or "go silent" that has the obvious effects.
Re-posting this, since I think this is very relevant. AI should not engage with loud weapons in stealth mode if they have the possibility to remain stealthy. Otherwise, "Stealth" mode makes little sense, since you do not have control over the AI's weapon.