Your stb features a stereo audio output so IMHO you do not need a multi channel setup. A 2.1 system mentioned above will suffice.
Most probably those red-white-yellow are RCA male jacks. The yellow one is for video and should be plugged to the TV. The rest red & white are the stereo audio outputs.
Since the audio is mainly for TV viewing, get the Logitech Z623 from Chadni Chowk, Kolkata. There is a exclusive Logitech Store in the E-Mall is Chadni Chowk. The store is on the ground floor.
Now the Z623 most probably comes with a RCA female to 3.5mm audio female adaptor, just plug that RCA male jacks of stb to the RCA female jacks(very easy since they are color coded) & then connect the 3.5mm audio input of the Z623 to the 3.5mm female of the adaptor.
These adaptors are also easily available in Chandni Chowk at Rs75.
Voila, and enjoy TV viewing. I am preferring Z623 since these sets are THX certified & are more optimized for movie/TV experience. I have the Logitech Z-2300 and it is too good for movies & games.