Applying effects to played sounds?

I'm wondering whether it's easy (or even possible) to add real-time changes to sounds played. For example, I want a radio to play a speech sound, but of course, for it to be a bit crackly. However, it's feasible that the player will hear the same thing said not through a radio. So I can either du-plicate every single voice file in the game, or find a way to add noise when the sound is played.
Another example is the modulation of engine pitch, which I'm pretty sure has been done before (in the CSK, if I'm not mistaken).