Hydra gets sick and tired of the Assert speaking, screaming and whimpering so they just end up removing its voice box to shut it up. Maybe they removed his tongue too by nailing it to a table or something and slowly cutting it off while the assert has to sit there and let them do it or it's squirming and trying to stop them for doing it.
+ If Steve feels horrible for not trying to save Bucky sooner when he realises that he's basically a mute now.
++ While they're cutting out the tongue, they're fucking the Assert so the cut is jagged and not straight to make it harder to heal.
Hydra takes Winter Soldier's voice box out.
Maybe they removed his tongue too by nailing it to a table or something and slowly cutting it off while the assert has to sit there and let them do it or it's squirming and trying to stop them for doing it.
+ If Steve feels horrible for not trying to save Bucky sooner when he realises that he's basically a mute now.
++ While they're cutting out the tongue, they're fucking the Assert so the cut is jagged and not straight to make it harder to heal.