'Oh, murder' just seems trite to me. If you could see the light through the gaps in the floorboards (correct me if I have that wrong), you could almost certainly hear everything that happened in that room. I don't think that there's a big conspiracy here, just ordinary people too scared to admit that what they actually heard that night was a murder, and they did nothing about it.
It may sound trite when in print, but it's not easy to translate what may have simply been a gasp or a shriek into a written word. There's a couple of reports that give it a little more context.
Daily News 13 Nov;
"Perhaps the most sensational bit of evidence tendered was that of a garrulous young woman who, with some dramatic force, imitated by voice and action a sort of nightmare cry of "Oh! murder!" which she declared she had heard just after she had been woke up by her kitten rubbing its nose against her face about half-past three or four o'clock on the morning of the murder. It was a faintish cry, she said, as though somebody had woke up with the nightmare"
The Echo 12 Nov;
And, just as I pushed the kitten away I heard, "Oh! Murder!" It was as if it was a nightmare. It was just "Oh! Oh! (in a faint, gasping way) - Murder!"
Incidentally, I don't think Mrs Prater lived directly above Kelly, but in a first floor front room. So the light she might have seen (but didn't) as she went upstairs would probably have been through the gaps in the partition wall, rather than through the floorboards. She did say she would have been able to hear sounds from downstairs, though.