This javascript app calculates a spectrogram from the input to your computer's microphone. (You may have to adjust the sensitivity a bit.) A spectrogram is in some ways similar to the activity pattern that auditory nerve fibers send to your brain, so you can use this spectrogram app to visualize what various sounds or speech would "look like" to your brain. You can pause the spectrogram by clicking on it.
Note that a spectrogram differs from the activity pattern in the auditory nerve in important ways. For example, it has a linear, rahter than logarithmic, frequency spacing, and it does not take into account that the frequency tuning of the inner ear is progressively broader for higher frequency fibers. If you want a more realistic representation of the tonotopic representations of sounds along the cochlea, check out our online cochleagram page for comparison.