Dear Sabreena,
you may use the Matlab component to load the voice signal into OptiSystem or you may load a digitized version of the voice signal (after sampling, quantization and encoding) as a text file to the “User Defined Bit Sequence Generator” component in OptiSystem.
You may find examples on using the Matlab component in OptiSystem at the following link.
C:\Users\USER NAME\Documents\OptiSystem 17.0 Samples\Software interworking\MATLAB co-simulation
Regards,
Ahmad