How to calculate biquad coefficients for the partial (1%, 10%) applying of pre-emphasis effect
So, sox defines its biquad operation parameters as follows:

biquad b0 b1 b2 a0 a1 a2



  Apply a biquad IIR filter with the given coefficients. Where b* and a* are the numerator and denominator coefficients respectively.
  
  See http://en.wikipedia.org/wiki/Digital_biquad_filter (where a0 = 1).
  
  This effect supports the −−plot global option.


A biquad filter is just a special case of an IIR filter, which is a linear system! 

That means that you can just have two signal streams, one filtered and one un-filtered, and add them up, scaled by two coefficients with constant sum.

The only thing you'd have to care about is delay, which is a bit tricky to calculate for IIRs; now, luckily, the RIAA biquad guarantees phase errors of this filter to be below 30% at any given frequency, so you can probably get away with summing up with a single-sample delayed input signal. 

A single sample delay is a FIR filter with coefficients 

b0 = 0, b1 = 1


and any FIR is also understandable as IIR with all aN==0 but a0=1, since IIRs are linear, you can directly modify the coefficients of your biquad by mutliplying them with the aforementioned coefficient and adding 1-coefficient to a0 and b1