If the speakers are removable (which most old stereos are like that) you should be able to get an adapter so that you can connect the speakers to the phone.
I once wanted to hook up an old tape recorder to my computer so I could copy the sounds from it (which there was no other way to obtain). Worked like a charm. Just have to connect them the right way.
Since you can use external speakers on a phone, mp3 player, computer, etc., many of which now come with 3.5mm plugs, as long as you can make that same connections with the speakers you have, then it should work.