Seriously how tough would it be to NOT add speakers and fake sound, but to voice-modulate the existing exhaust sound coming from the tailpipe into a lower octave or pitch? The flamers here are not contributing to anything but the George Bushing of America. Great comment; less dumbing.
the exhaust system is a mechanical system. not an electronic system. you cant electronically alter the sound coming from an engine and out the tail pipe. its not a speaker.
you can alter the sound by say, adding resonators, different mufflers, electronically controlled dump pipe etc etc. or designing and installing a active exhaust system like the ones aston martin and bmw use. that only get loud when your on the pedal...