I am a newbie, so take it with a grain of salt.
In low light situation, autofocus does not come immediately, and when you let the shutter go before the AF is locked, you are asking the camera to take the picture anyway (even if it is not focused properly). By looking at the two pictures, you are making the camera work hard since you have it in AV mode combined with spot metering. Also, the AF point on the first picture is on the person's dark shirt on the left; while the AF point on the second picture is on the person's hat in the back rather than his face.
Try the same setting in manual mode 1/30 and F4 - let the flash adjust the output and give the camera a break. Also, in the first picture, I think it is a focus error; in the second picture choose the AF point closer (e.g. front row lady in the middle) and put the AF point on the face - something which is more contrasty and easy to get focus lock. With FLs at 16 and 24mm, the DOF is pretty wide, so everyone in the picture should be in focus. Again, without the focus confirmation from the camera, you should not expect sharp images at all (I think it is a reasonable assumption).
If it does not make sense to you, please ignore me.