[Content warning - discussion of physical, emotional, and sexual abuse - mention of homophobia, transphobia, and biphobia]
When people think of violence or abuse, they tend to think of that stereotype of physical violence - aggression and anger. Silence can seem like the antithesis of that - calm and peaceful. But silence can show up in many ways in abusive contexts, sometimes playing a key role in its continuation.
One of the most pertinent ways that silence can impact on abuse is when abusers coerce victims into silence. This can be done in a wide variety of ways, such as making threats about what they will do if the victim tells someone (usually threatening to harm someone they care about), or shaming them into silence. This can involve using the victim's own actions against them, like those who have been exploited online being threatened with the exposure of those images to family members, friends, employers and so on. For LGBT+ people, it can include being outed without your consent.
When victims are silenced through fear, coercion, intimidation, and further violence, it often enables the abuse to continue. It first accomplishes this by making sure that those who would be likely to intervene are not aware of it. It can also function later as a means of invalidating the victim's experience - such as people questioning why the victim didn't tell anyone sooner.
...if the abuse is chronic then they could have learned that silence was a better coping and survival strategy.
The concept of Learned Helplessness refers to when people are repeatedly exposed to a stressful situation or occurrence and as a result, feel like they are unable to change or control it. For instance, if an individual was to be hurt once by a perpetrator, they may fight back, call for help, tell people afterwards, and take action to avoid that person. But if that individual was hurt repeatedly by that perpetrator over a long period of time and the situation felt impossible to change, they might stop fighting back, and they may fall silent - in the belief that such actions would either serve no purpose at all, or they may make it worse.
This is why it can be problematic when people ask how much a person resisted during sexual abuse or if they called for help - because not only can some people freeze or dissociate which makes such actions difficult and/or impossible - but if the abuse is chronic then they could have learned that silence was a better coping and survival strategy.