I think the OP is being ridiculous. I have brought entire disassembled desktop computers, complete with 10 hard drives, graphics cards, memory, CPUs, and even a large LCD monitor through checkpoints with no problem or serious hassle. Since the carry-on does look suspicious they always did a thorough search of the bag post-xray, which delayed me by maybe 10 minutes or so, but it was no big deal. As long as you have the space in the bag bringing a car stereo with you should be pretty easy. Although you should of course expect an additional 5-15 minute delay while they search your carry-on. The problem arises when you have something that really does look bomb-like. I don't think a car stereo qualifies.

Also, outside of a handful of countries international shipping is generally quite expensive, and often unreliable as well. Very often it makes more sense to sell the item locally and buy a new one when you arrive instead of shipping it.

If you don't completely trust him you could always take apart the stereo if it doesn't look too complicated or ask him to do so before taking it. If there are drugs in it it would be pretty obvious. I normally wouldn't be able to do that sort of thing even for a good friend because my carry-on capacity is almost always 110% full, with me even having to try to carry stuff in my pockets etc. It sounds to me like you actually don't trust this "friend" of yours so much. In which case, it's not worth the hassle anyway.

BTW, the story as you have posted here makes no sense to me. Why can't he just bring it back with him in his own carry-on when he returns? Is he not planning to return perhaps because the FBI will arrest him on arrival? And he bought it in the states, had it shipped internationally to him while traveling (for some reason that I don't understand) and then doesn't want to ship it back again? Very, very confusing story. My above advice pretty much assumes that it only sounds suspicious the way you are telling it and that in reality it all makes perfect sense if explained properly.

Otherwise it sounds like your colleague doesn't care whether you go to prison for a long time and is looking to make some extra cash. You might have considered saying yes so that you could open the thing up and see whether there was a package of brown/white powder inside of it. Then you would know for certain that your colleague was in fact a drug smuggler. I wouldn't turn him in (ratlike behavior), but I would never talk to the person again if they were trying to use me as a free mule.