That's an interesting question -- I have thought about this in the past...

Two orthodox solutions;

1. Discrete IC (yes they do exist)
2. Chain of switchable attenuation resistors

With the resistors, ideally you'll need to formulate something that translates into db. Better still; have the LCD or LED displays show the attenuation level in db. At the end of the day, a lot digital circuits that control audio attenuation induce two undesirable effects into the signal...

1. Harmonic distortion
2. Crossover distortion (bad channel separation)

For superior audio quality; hard-to-beat the good'ol humble POT. Watch out for bandwidth too. I think some of these circuits make an otherwise excellent preamplifier mediocre.