Except for Atlas products, most HO diesels have the body held onto the chassis by the two screws holding the couplers in place. If you remove them the body should come off.
MRC's "Sounder" is a good choice for a piggyback to the regular decoder. However the Bachmann engines come with decoders that aren't all that great so I'd suggest going with a combined sound and control decoder. MRC combined decoders get a lot of bad reviews on forums like this so I'd maybe skip their products, but several companies like QSI, Soundtraxx and others make good sound decoders you can use.
I'd maybe start by checking out Tony's Train Exchange, besides having the products for sale they have a lot of info on DCC and DCC sound that will get you started on the right path.
http://dcctrains.com/tonystips/dccprimer/index.htm
http://dcctrains.com/products/sound.htm
I am new to model railroading and purchased a Bachmann Spectrum series HO 8-40CW Loco-DCC Equipped. My goal is to add sound to this locomotive, but I need some help with that. First I have serached the internet for instructions on how to take this loco apart. I need help pictures would be even better. Secondly the reason for me wanting to take it apart is to add onboard sound. And last thing would be what sound decoder should I go with I do not want to spend alot of money but I want something reliable and easy to operate on my Bachmann EZ-Command. And should I go with something like MRC decoder that uses the existing DCC decoder or should i buy a all in one package.
Please help my kids would love to be able to hear sound as I run the train.