Fitting sound to Bachmann Spectrum 0n30 railcar and trailer

[Illawarra73]

I have recently been introduced to the joys of DCC and am going to build a 0n30 layout with a friend. I have acquired two locos both Bachmann, a Porter with DCC sound and a Railcar and trailer DCC only but read somewhere that it could be upgraded easily to sound. I have been surfing the net but cannot find any details on what number decoder and speaker to buy all the information is just general that it can be done. As a "newby" I do not have that knowledge that a lot of others do and living "downunder" in Australia a lot of items are not readily available or the people with the know how so please is there anyone who can guide me in what I need to achieve my aims.
Many thanks in anticipation.