harmonic algorithm for extending compressed bandwidth of audio file
there is also this answer in this comp.dsp thread.


high-pass filter your input signal (keep the unmolested input around for later).  the cutoff frequency should be adjustable.  i would think a 2nd-order HPF with a Q approx= 0.707 (Butterworth) would be good enough.
square that HPF output,
add (maybe with an adjustable gain coefficient) that squared HPF back to the original input and output that audio sample.


see if that brightens up your day.