I know the feeling. I will say there is nothing external locking your vocal cords other than you. The reason it feels like you can't is because toy are still prioritizing not stuttering over getting on the sound and taking that risk. It feels involuntary but that's what it is. It's a big chance to take and one you've probably never taken before. In order to work on overcoming the instinct to block, you gotta start at the easiest/safest situation where you block and tell yourself that you will make a feel sound and be prepared to stutter and possibly feel awkward and make an embarrassing sound in order to get on the feared found instead of blocking. Maybe you will make the sound for one second and then go back to blocking. That is still mor progress than before. You need to do this though in as safe of a situation with whoever you feel most comfortable doing this with. Does it make sense?