Is it possible to steal processing power or memory space from other components in a computer that would otherwise be unused in the program, such as sound/firewire cards etc, or can only the system/graphics processors and memory be utilised?

It is possible if the hardware provides that option, for example you couldn't with older GPUs, but with newer cards you can use the GPU to help the CPU (using shaders). It's not as simple as calling a single command, if your hardware supports it, it will most likely be something like programming shaders on the GPU, but with less support and documentation. Also I doubt that sound cards have enough processing power for it to be any faster (it will most likely be slower since you have to transfer data over the BUS), do the sound card even have a processor? For normal applications you should stick to the CPU and maybe the GPU, but remeber that you will have to change the way the code works to make it take advantage of the GPU (transferring data is very expensive).