Tkinter combobox - gracefully lose focus when click out of widget
You might want to try this:self.hcdr3_length_combo.bell(displayof=1)
Not sure if it should be 1 or 0 though...If it doesn't work, maybe a containing widget throws the sound. Might want to apply it to the parent widget as well. I'm not familiar with python 2.7 and it doesn't throw a sound when I use python3 with slight modifications.
Usually when you can't find an option for a specific widget, you can find something in the general widget options. Just search "tkinter widget options" and you'll get some place like:https://effbot.org/tkinterbook/widget.htm