Can you suggest any alternative audio mixer IC for SSM2163
Unfortunately we don't have a direct replacement for SSM2163, But in case your interested on other option that uses a discrete solution, you can look at the SSM2019. The part is a self contained pre amplifier with noise level at 950pVrtHz and THD at <0.01% at G=100 over audio band. The part can be used as a very low noise bus summing amplifier which may not be completely identical with SSM2163 but if designed together with digital potentiometer at the input, that would act as a digital controlled input attenuator, it can also be used as a digitally controlled audio mixer. But please be minded though of the input and output impedance which is higher compared with SSM2163. I am attaching the link of SSM2019 DS and selection guide for Digital potentiometer for options in resistance and number of channel.
Can you please share more information on the intended application of SSM2163 maybe the project that you are working that uses the audio mixer.