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; obtaining, by the processor, a low-frequency training sound signal; estimating, by the processor, a filter model characteristic based on the low-frequency training sound signal; determining, by the processor, an inverted filter model characteristic; and combining, by the processor, the inverted filter model characteristic with the preconditioned internal speech signal to produce a preprocessed internal speech signal.