In an area that small it isn’t really that hard to distinguish. Someone doing the banging would be making physically exerting noises in the process. Sounds of a scuffle are fairly distinguishable and I doubt anyone will simply go limp and let themselves be smashed into walls. Even if they are after the lawsuit lottery. The body will simply react automatically to defend itself.
Perhaps sounds of a struggle would be evident - But I think it more likely the sounds of a struggle would more closely resemble a struggle if neither party was in handcuffs and leg shackles, though.
One report tonight was saying that he had not been strapped into his seat which was a relatively new departmental mandate. Also, the other prisoner may be recanting.