I recomend using an lm386 audio op amp to handle the amplification. If you boost the gain up by connecting a 20uf capacitor accross pins 1 and 8 you may get some distortion at the ouput to make your voice sound like it is coming over an rf link. You could also use a different op amp (lm741 ect) and wire it up with a very high gain to increase distortion. This could be achived by conecting a low value feedback resistor from the inverting input to the output. After you amplify the signal, you should feed it into a comparator chip. An lm393 would work well for this application. Set the + threshhold voltage with a two resistor divider. Conect the output from the audio amp to the - input of the comparator and pull the output of the comparator up with about 3k. The ouput can then be fed directly into the PIC. Your program will simply need to wait for the output of the comparator to go high and then fall low again indicating that you have finished speaking. It can then be programed to output white noise with the sound command. PIC generated white noise sounds very steady. If you want a more realistic sound you could try to set up a very simple radio reciever built around another 386 op amp. This is simpler then it sounds and would produce a good sound. You would simply wire up the lm386 as usual and then conect a tuned circuit (capacitor and inductor) accross the input. You could easily wind a simple coil and use a small value capacitor to make this circuit produce the desired sound.