Which files should I modify to get rid of the unpleasant buzzing alerts?

I accidentally figured this out while playing around with the Dev Tools.

• Enable Dev Tools in the Settings menu

• At the top of the screen, click the third icon from the left in the floating menu (two overlapping squares)

• Click SoundDefs, No package loaded, UI_Oneshots_Misc

The sound you're referring to is called LetterArrive_BadUrgent. You could delete it by pressing the X (I didn't try it, so that may cause errors), or click it, then adjust the volume using the volumeRange setting.

Note, once you click on the sound name in the menu, you can listen to the sound by clicking the triangle play button at the top of the window that comes up.