I also use the Audacity Software and it works great for adjusting the Volume levels and cleaning up those Popping Sounds in some of the assets.
The reason the Bells have Double Sounds is because there are 2 Bells for one Grade Xing and they will some times sink and you do not hear that Double Sound.
BNSF had the Double Sounds, Maddy"s Sounds had a Single bell in her crossings. I have uploaded a huge amount of the BNSF Grade Xings to the DLS under the Name jjeff1955, Jons Grade Xings, with Permission from Clem's Wife to do so. and most of them have the Double Bells built in, and I could not remove one of them to stop that Double Sound.

In real Life Crossings do have double Bells one attached to each Light.
The only way to stop the Doppler effect is to turn off 3D Sound in the Game;s Audio Settings.
But then there will be no panning effect from the Stereo Speakers.
All Custom Sound files have to be in Mono, not in Stereo, or the Volume will not be controlled by the game's Audio Software.
I have gotten use to the Doppler effect and do not notice it so much anymore.
Hopefully, maybe in the Surveyor 2.0 they will have a way to just turn off that Doppler effect as an option.
I hope this helps you under what you are experiencing with the Bells.

Cheers!