The last release of OpenBSD (4.8) includes a new cool feature: a monitoring mode in its audio daemon aucat(1).

Thus, this little hack visualizes anything, what is being played using this new mode, no matter where the sound data came from.

Source: src/spectrogram (depends on sndio, SDL, fftw3)

UPD: aucat was renamed into sndiod. To get it working restart sndiod with flags -m play,mon,midi