I don't understand what the white cam lead gets hooked up to.
Some model steam locomotives are equipped with, or can be equipped with, a cam arrangement which coordinates the sound to the rotation of the drivers creating chuffing.
As the wheels turn, a switch opens and closes, operating the chuff sound produced by the sound decoder.
The white wire goes from the decoder to a microswitch which is operated by a cam that is mounted on the axle of one of the drivers.
In operation, when the white wire senses the DCC track signal, it operates the chuff sound of the sound decoder.
For locomotives that do not have this cam arrangement, the circuitry and programming of the sound decoder produces the chuffing sound based upon the speed and load conditions of the locomotive.
This is not as precise as the cam controlled sound, but in most cases it is not noticeable.
In many cases, this "microswitch" arrangement is a simple phosphor bronze wiper which presses up against a synchronization cam that is mounted on one of the locomotive's drivers.
The cam is a piece of printed circuit board material with electrical conducting foil at periodic locations.
This cam is then grounded to that driver, completing a connection to the rails.
For a typical steam locomotive, there are four segments of foil.
For a Mallet type (or simple articulated), there are eight conductors.
For a Shay or other geared locomotives, there are a larger number of conductors.
In operation, as the driver turns, the microswitch periodically completes a connection between the wiper and the cam; the faster the driver turns or the more conductors, the more circuit completions and thus more chuffs.
Some locomotives, especially in the larger scales, may have a microswitch which presses onto a mechanical cam which is mounted onto the driver axle.
Again, as the wheel turns, this microswitch is opened and closed in concord with the actual speed of the locomotive.
In this case, the white wire is connected to one side of the switch and a return wire goes back to the decoder.
Needless to say, this is not necessary for diesel locomotives.
Sound cam circuit boards are available from third party vendors.
And, this additional bit from AJ: For the Digitrax sound CAM input signal, all that is required is a source of voltage that goes "on" (above about +6V) and then "off" (no voltage for e.g. 10mS). The actual DCC track side is not critical. The only issue would be for DC operations when the track connection in one direction makes the signal always negative and unreadable. For operation on DC then, the cam signal will need to have a +ve power supply level, as e.g the new Bachman K27 seems to have.
If you do not wish to hook up a cam to the white wire of the locomotive, simply use the built in autochuff feature by programming CV133 to a value between 1 and 127 and CV134 to a value between 0 and 32.