One reason for an electrical modification (rather than muffling the speaker) is explained in this OGR thread from just last year. Apparently the electronics will overheat and fail if powered by 18V AC. As mentioned above a voltage converter should be considered irrespective of your volume control. The following is from this OGR thread which may have a few more tidbits applicable to your situation:
But specific to volume. I believe, as suggested by others, you will achieve some level of volume control by using the Buck Converter module and adjusting the voltage going into the bell sound module. My guess is as you lower the voltage below, say, 7V DC you will hear the volume drop. Above 7V and the volume will be steady. Then if you go down to maybe 4V DC it will just stop.
I'd try the above first. If this does not give you the adjustability you want, then I think you have to open the sound module (see the photos from the first thread I linked). Then you will need to install a component between the electronics and the actual speaker in the module to reduce/adjust the volume. It's not that this would be expensive (less than $1 in part(s)) but it would involve opening up the module and possibly require some soldering...
