Ummmmmm..... yes and no.
If you were to hypothetically wire your speakers directly into a 3.5mm headphone jack, plug that into an ipod/iphone, and press play, you might have sound. The jack on your mobile device does not push very much current, so the likelihood of you getting much out of a 2.1 sound system is unlikely. While you could run a pair of 5-10w speakers (I bought a pair for my ipod many years back), the only real reason to do this is because you have no power source, and want you music louder than through the built-in speakers.
If you were to plug your ipod into a 2.1 system, that was designed to run on a/c power, you'd experience very minimal volume and sound quality, at best, if you managed to tap everything directly. Your tablet will be no better, as you're using devices that are made to power headphones, which have an output in the milliwatt range. For reference, a milliwatt is one thousandth of a watt, and your typical speaker system can range from between 20-150 watts. Your device isn't pushing enough current to make a quality sound, if any at all. Even if you plugged your device into low wattage speakers (like the ones I bought many years ago), you wouldn't even be pushing the speakers to a point where they are performing at a level that is remotely decent for their rating.
Short answer: hell no, this just wouldn't work out. If you heard anything at all, it would be at a low volume, and you would have very low quality sound, at best.