I agree avatar, and only mentioned such to show that the suggestion (was there even a suggestion?) was lacking in completeness.
Another alternative solution to why the bard isn't noticed, but the 'stabbed mob' is alert is that when they are wounded, they are alert! Someone hurt them, so they are being careful. Along comes happy tune th/ba, singing along a happy melody, and 'wham' out of no where, stabs the mob. He noticed the th/ba, but just not the blade getting stabbed in his side/back/face/whatever.
I think the current system is fine as it is.