Jamie
In moulding jargon, what you are after is called a 'core', it is a seperate moulding (positive) of the void you require, which is then fixed in the primary mould cavity. These cores have to be made out of something that is dispensible/removable and in days past sand was used in most moulds, but today they probably use styrene or something. For your purposes, I think I would use bees wax, so you will have to determine the shape/area of this 'hollow' and make a pattern of the void required- I would not bother with all these fancy moulding materials, just use hobby clay (plasterscene), soften it up and shape it to what you need and then freeze it- remove it from the freezer, and then make a positive (2 piece mould) of it with whatever medium you prefer, let it set, remove the hobby clay - start pouring, make more 'cores' than you think you will need, then fix one in place in the main mould (you will have to work out how you are going to fix it in the main cavity), the master mould must be jigged to fit each new 'Core' back into precisely the same place every time you want to cast a lure, this way every lure will be the same (same as casting a hook into a jig). I say use wax because when the lure is cast and is removed from the mould, this 'core' will be stuck in there, so you will have to melt it out leaving your sound chamber. Whatever material you use for this 'core' should be disolvable/removable weather you dig it out or dissolve/melt it, so maybe you can come up with something!! sugar dissolves in water??. The best of all if you can shape it would be styrene foam, just pour petrol on it (or spray 'Carby Clean') an it dissappears. For a more hi/tech up to date material, 'Reeves' (George) 'dlaery' (David) may be able to help, they are all over these new silicones etc. Sounds easy!!, I just finished a swimbait and I thought that was hard, phew. pete