Well I would not suggest placing speakers behind anything less than a completely acoustically transparent fabric. You could still use that velvet for other parts your false wall as far as I know. As far as the light absorbing quality of that particular velvet I am not sure. There has been several threads discussing the merits of triple black plush vs protostar vs velvetyne and many others.
I am still unsure which black velvet I will use for my screen frame. Also not sure if I will use velvet for other areas of the false wall or go with something like gom fr701. I am even considering using speaker fabric for the part of the false wall that covers my mains. Unless I can be convinced that something like fr701 will not be detrimental in any way.