hehe, this is a can of worms - what sound(s) exactly do you want to recognise...
Single frequencies aren't too bad, you can find and download a FFT algorithm for VHDL from (Home :: OpenCores), once your in the frequency domain you can apply various filtering etc to try identify whether what you are looking for is present in the input signal.
Beware, simply knowing VHDL is not enough to solve this problem with ease... do some reading on discrete signal processing, discete LTI filters, Fourier transforms and the FFT algorithm. you should find articles to most of these on wikipedia. I would start here, and then continue on to do some more reading if your brave enough