Steganography is the process of hiding information on a host signal. Transparency is referred to the abilityto avoid suspicion about the existence of a secret message. The most popular mechanisms for hiding datain audio signals are the Least Significant Bit (LSB) substitution, Frequency Masking (FM), Spread Spectrum(SS), and Shift Spectrum Algorithm (SSA). In this paper, we adapt the Frequency Masking concept using anefficient sorting of the wavelet coefficients of the secret messages and use an indirect LSB substitution forhiding speech signals into speech signals. The experimental results show that the proposed model, theEfficient Wavelet Masking (EWM) scheme, has a hiding capacity significantly higher than the Spreadand Shift Spectrum Algorithms and additionally a statistical transparency higher than all of the abovementioned mechanisms. Moreover, the transparency is not dependent of the host signal chosen becausethe wavelet sorting guarantees the adaptation of the secret message to the host signal.