A "normal" stereo, as in an aftermarket head unit pushing aftermarket speakers in factory locations will draw between 2 and 6 amps of current at normal listening volumes, peaking as high as 12 or so amps under the heaviest of loads, momentarily.
This isn't much at all, nor is the weight significant - I would consider its impact on fuel consumption trivial at best.
Throw a modest subwoofer and modest amplifier in the mix (not the kind of thing you hear 6 blocks away... just adequate augmentation to a normal stereo) and you're still under 10 amps typical current consumption at less than full volume listening.
The guys who are really serious about car audio might have systems drawing over 250 amps of current and subwoofer enclosures weighing over 400 lbs.... but that's probably not what you were asking about
