I recommend you call Bruce at Litchfield Station. He is very knowledgeable and is careful to listen to your needs/wants rather than just trying to sell you something. Since they do a lot of installs he will probably have just the answer for you. Friends tell me the folks at Tony's Train Exchange are good, also.
Good luck!
For no sound, a TCS A4X decoder would be a very easy install by replacing the Athearn light board.
Information about the TCS A4X can be found here:
http://www.tcsdcc.com/Customer_Content/Products/Decoders/HO-Scale/HO%20Series.htm
For sound, the main problem you're going to be faced with is finding room for a speaker.
A QSI Revolution sound decoder is practically the same size as the TCS A4X but you still have to find room for a speaker and the rather large capacitor that is attached to the decoder, so the first thing you need to do is open the model and see where or if a speaker can be fit into it.
Here's information about the QSI Quantum Revolution sound decoder:
http://www.qsisolutions.com/products/q-revolution.html No matter which decoder you choose, be aware that Athearn Genesis locomotives use 1.2 Volt incandescent light bulbs, and if the decoder does't have built-in resistors for these bulbs you will have to add your own.