This paper applied the high quality speech coding algorithm G.729 which has been standardized by ITU-T with low bit rate 8kb/s. This algorithm is based on a conjugate-structure algebraic CELP (CS_ACELP) coding technique with 10ms speech frames. The output of encoder is encrypted by a symmetric key Blowfish algorithm which has 64 bits block size and a variable key length from 32 up to 448 bits. One of the main disadvantages of Blowfish algorithm is the time required to initialize the algorithm with the key. This paper proposes a new method for generating S-boxes and P-arrays which are considered as the main building elements of the Blowfish algorithm. This new generating method leads to a reduction in time complexity of generating S-boxes and P-arrays. The proposed speech encryption system has been implemented using Matlab and the output is analyzed using Avalanche effect. The results showed that the modified design of the algorithm continues to offer the same level of security as the original Blowfish cipher with a less computational overhead in key generation.