how much "sound inputs" of high fidelify (128K 44kH) may be made via ordinary FPGA (Xilinx Spartan 3, what-so-ever) without using external ADC converters (only voltage-balanced input optically taken from audio jack)?
Here, Generating a pure sine wave as output form FPGA using VHDL code , it is pointed that FPGA may not perform such a job itself.
However, there are still 4 inputs/4 outputs of this resolution in the proposed test project, and...
what is a possibilty/amount of gates required to implement pre-DAC/pre-ADC output?
No, you can't take analog signals. All Xilinx FPGAs are fundamentally digital. You need the ADC.