I want to know that if I want to make the sound polyphonic, should I add a multiplexer or it is programmable.

please give me some guidelines, brothers. thx

Depends on what you mean by "the sound", but generally polyphony is a function of software if software is generating the sound. There is not a lot of scope for this on an arduino though.

I want to produce a piano which can play a chord...(or 2 NOTES at the same time...)

bhagman from this forum created a tone generation library that will play more than one tone at a time through software.

See http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1251750920