There are two that sort-of fit the purpose, Ironcope and osc. Both designed originally as pretty visual effects for audio playback. The former is a home screen widget, rather small but can show the stereo waveform. The latter is full-screen but with a rather confusing interface and the waveform it draws is a bit too decorative to be of actual use, but it does give some idea.

I wanted to grab the source of osc and adapt it for a proper use as an oscilloscope but it is in the non-free section, i.e. without source code. Another option is to take Ironcope and turn it from a widget to a full-screen app. That is something I might still do but, with my current lifestyle, it might take a few months if not a whole year.