1. A method for preprocessing speech signals received from an indirect conduction microphone, the method comprising: receiving, by a direct conduction microphone, an external speech sound; estimating, by a processor, an external speech spectral model based on the external speech sound, the external speech spectral model including a plurality of coefficients; receiving, from the indirect conduction microphone, an internal speech signal; combining, by the processor, the plurality of coefficients with the internal speech signal to produce a preconditioned internal speech signal.