Things can often sound worse than they are when heard through the wall.
If she is being hit though, then she might or might not welcome help, depending on what point in the abused person's cycle she is at.
If she's still at the stage of believing that her dh is marvellous, and she is an unworthy being who does such stupid things that he gets angry, then she may reject your help.
On the other hand, if she's near the end of her tether, then she may subconsciously wish for help.
If you call the police without her cooperation, as she is very likely to swear that she wasn't being hit. In that case, I guess the partner would get a shock, but he is basically safe unless she cooperates with the police, which is a very scarey thing to do, and a very big step for someone who is being abused. She needs to reach this point herself, before she can rescue herself and teh children.
In your situation, I would try to speak to her alone, and say what you're hearing through the wall. I myself have been through the shit with a violent husband, and would use this experience to try and instill the idea into her head that she doesn't have to put up with violence, and that moreover, you aren't prepared to put up with hearing it (ie you will call the police in the future), and that you aren't scared of her partner.
I know from my own experience, that even if she says completely the opposite, the seed will have been planted, that she can (a) talk about the violence because it isn't private and (b) can get out of it.
She may tell her partner about the conversation, so you have to be prepared for that if you speak to her.