I find matching the volume levels of sound decoders individually by ear between one locomotive to another type locomotive difficult. The difficulty increased when the programming track is at a computer desk in a separate room from the layout where the final sound volume is heard during operation.
Matching volume levels between several locomotives with the same sound file downloaded into the decoder is not difficult with a computer and programming track where the locomotive sounds can all be adjusted to a same volume level that is pleasing to ones liking.
It is the difference between the many other locomotive type sound files in use on the layout that offer the most difficulty matching sound volumes for me; Alco 539, 244, 251 as an example and then there are EMD's and motors too. Not only prime mover sounds but I like to match horn, bell, compressor and other sound levels as well.
I have started to use a sound level meter app in conjunction with the computer and programming track to even out the all the sound volume levels of the layouts many different type locomotives. So far this has worked far better that by ear!
The app is free and downloadable from The National Institute for Occupational Saftey and Health (NIOSH). The app can be downloaded here.
Below are a few example photos of the downloaded app in use with a cell phone on the layout, the meter numerals increase or decrease as sound levels rise or lower.
Alco 244 at idle,
at idle with horn,
at run 3.
Maybe this app will be helpful for others?