Shahzad:
The relay switching idea is probably your best solution if you cannot design the OP-AMP mixer circuit. Personally I think the mixer circuit idea from the last thread is the very best idea. Keep in mind that it may very well be necessary to put an amplifier after the opamp to drive the speaker with sufficient amplitude. But there are even opamps that have outputs fully capable of supplying enough current to drive a speaker but LM386 is a good cheap solution.

There are chips called CMOS analog switches that may be usable in your application. If you google a bit you can locate some "high current analog switches" that can handle currents up to 80 or 100 milliampers. (Maxim has at least one part). Keep in mind that analog switch ICs have specific limitations as to the range of voltages that can pass safely through them so check the data sheets carefully.

Michael Karas