A joint spectro-temporal auditory model is utilized to assess speech quality objectively. The model mimics early and central auditory functions and serves as a spectro-temporal modulation filterbank. Three perceptual relevant parameters, intelligibility, clarity and naturalness, are addressed by the model and are combined to estimate the subjective mean opinion score (MOS) for speech quality measure. Through a simple multiple linear regression analysis, we demonstrate the performance of our proposed perception-based objective speech quality measure is better than that of the state-of-the-art P.563 standard in estimating MOS of the codec-distorted speech in ITU-T Supp. 23 database.