SoundPads by Sonic Impact are stickable little pads that “convert any thin-walled high-transmission surface into permanent stereo speakers.” That means you can stick these guys on a door, cabinet, table, box, desk, or even a poster- and have insta-speakers.
The site gives you fair warning that once these are stuck, they aren’t coming unstuck. Where you stick them affects the sound quality, of course, so you might want to do a bit of trial and error before attaching them permanently to your vintage Q*bert poster.
The SoundPads are just over 4 inches in diameter so should remain relatively discreet. They promise to “make most surfaces thump with sound.” I like the sound of that! You can get a pair for $17.99 from ThinkGeek.