If you want to amplify audio from your device (like an iPod or smartphone), plugging it into the “AUX” jack of a nearby radio is probably the easiest way. Actually visualizing the waveform that you’re listening to is a little more difficult.
You could hack something together, but the Pretty Happy amplifier (PHAP) is available to do this job in kit form. It takes input from a standard 3.5mm audio jack, then amplifies it to a maximum output of 15 watts on each channel. This signal is then displayed on a tiny OLED display.
Although you could perhaps integrate it into an existing audio system, the board seems to be begging to be put in a see-through enclosure for all the world to see. Making it even more interesting, is that the OLED is controlled by an ATmega328p microcontroller, so if you’d like it to display something other than the oscilloscope or indicators that it comes with, you can modify it as needed.
You can see even more information about this board on its Hackaday.io project page.