Using Fast Convolution to apply HRIR to soundwave
Google "overlap add". Quick outline


zero pad filter to 256 taps, do an FFT
Initialize and "overlap" buffer of length 128 taps to all zeros
Initialize an input pointer to 0
Loop starts here
Take 128 samples from your input starting at input pointer. Zero pad to 256, FFT
Multiply the FFTs
Inverse FFT, add overlap buffer to the first 128 samples, output these samples. Put the second 128 samples into the overlap buffer
Advance your input pointer by 128 samples and go back to step 5. Repeat until the input is all done


Some quick tips:


Overlap save is slightly faster but a little less intuitive.
If filters and signals are all real valued you can speed things up further by either implementing a real valued FFT that's based on an N/2 complex FFT or by combining filters and/or signals in pairs
128 is probably close to the break even point between FFT based and direct FIR filtering. I wouldn't expect huge gains. Depending hardware and level of code optimization it may actually be slower.