Just to elaborate on what John said:
Pressing the Vol ^ or Vol v raises and lowers the "overall volume" which includes the whistle.
Pressing AUX1- Vol^.. AUX1- VOL ^... and so on will raise the chuff and other background steam sounds.
AUX-1 VOL v ....AUX -1 VOL v.... lowers the background sounds
Remember, you must pres AUX1 first before each Volume up press or the volume control will revert back to overall sounds.
On a side note. I had the NYC version, and these do have a weaker chuff than most Legacy engines.