Now that I think about it, volume adjustment comes up for other accessories... Mel's Diner comes to mind. If you're lucky you might already have or stumble across a suitable inexpensive potentiometer; but as shown above it might be a $5 part (plus shipping). For example, let's say your accessory has an 8 Ohm speaker (very common in accessories). If you simply add a 100 Ohm potentiometer between the electronics and the speaker, this will provide a volume reduction range of 0 dB (no reduction) to about 23 dB. Everyone knows dB relates to sound, but very few know a 6 dB reduction if it hit them in the ear. 
1 dB is about what you get when you add or subtract one bar on a TV volume bar meter. I don't know how "annoying" the crossing bell is, but my guess is you're looking for a reduction of at least 10 dB.
Note that with the potentiometer, you don't get "mute" capability with a potentiometer. Of course you can add a brute-force on-off switch which is a hard mute. 
Or, if willing to give up variable adjustment and live with, say, OFF-LOW-HIGH volume you can use a 3-position switch which gives you true MUTE capability in addition to 2 other volumes. As illustrated above (by way of example), if you use a few low cost resistors, you can experiment and pick what you want for the low and high volumes. As shown, the 3 toggle switch positions would be MUTE, 0 dB (original volume), 15 dB reduction. Or choose MUTE and any combination of 2 other reductions. You might even save a few bucks going this route.
I purposely skipped over the math of how dB reduction is calculated...involves logarithms and other eyes-glaze-over techno-babble. 
