Shahzad,

I was in the impression that you were making a one-off hobby type thingy. If this thing goes into production, I would strongly encourage you to do some more research and learn more about analog electronics as Michael has suggested in his post. There is a lot of info out there on the internet, even on analog electronics.

Personnaly (although being only a starting hobbyist as far as the 8051 is concerned) I would not even consider a relay as part of a solution to your problem. It's a lot better to use some analog transistor -or opamp based mixer. It would also be to the benefit of your design and customer.

rgds
Patrick