When the lobby is enabled and the moderator uses an ios device with safari (not the app), then there is no knocking sound.
I suspect, that this is related to what I wrote in Lobby knocking sound issues
" When my pc has different loudspeakers enabled to what I have selected in jitsi, then the knocking sound gets played by this device"
At least this is true when using windows and jitsi in a browser.
Somehow the knocking sound (and maybe other jitsi sounds like join or leave) uses a different audio configuration than the conference itself.
Not a big issue but improvement would be nice.