It sounds (pun intended) as if you have "Clikity Clack" turned on. Thsi feature is turned on and off from the Sound menu.
From page 34 of The DCS O Gauge Companion 2nd Edition:
CLIKITY CLACK:
• Press soft key S1 (OFF) to turn clikity-clack off
• Press soft key S2 (AUTO) to set clikity-clack to automatic operation
• This will cause the active engine to enter clikity-clack mode of operation after running at 30 SMPH or greater for 30 seconds
• Normal engine sounds will cease
• The bell and horn or whistle become inoperative
• The engine will make the clack-clack sound of riding over rail joins
• When engine speed is changed, clack-clack will turn off and normal engine sounds,
including bell and horn or whistle, will return until the next time that the engine runs at 30 SMPH or greater for 30 seconds
Also, refer to the top of page 166:
Next, check DCS Menu settings. In the SOUND menu:
• Set Clickety-Clack mode correctly. This will turn off all other engine sounds when set to Auto and engine runs at or greater than 30 SMPH (approx.) for 30 seconds
This and a whole lot more is all in MTH’s “The DCS O Gauge Companion 2nd Edition", available for purchase as an eBook or a printed book at MTH's web store!