I don't have a CW-80 but I made this short video which shows what to expect. This is an MTH Z-750 controller which uses a similar transformer "technology" as the CW-80.
The voltmeter is a low-cost, hobby-grade meter that you can get FREE (with coupon from Sunday newspaper) from Harbor Freight Tools. 
The point is to show that you don't need a fancy-dancy, professional-grade meter to make this measurement!
In this case there is negligible (less than 0.1V) DC voltage when no button is pressed. With Horn/Whistle you get several Volts of positive DC (albeit no "+" sign shown on meter); with Bell you get several Volts of negative DC ("-" sign shown on meter). This DC voltage does not materially change with variations in AC track voltage...note the brightness of the lock-on lamp changing as voltage control is rotated up and down.
The video has sound; in this case it's an MTH PS2 engine you're hearing. But the mechanism of detecting the DC voltage on the track to activate Horn/Whistle and/or Bell is the same as your Sound Cards.
