The Safari browser that is built into Apple devices will only sound the audible chime if the tab is NOT active. The tab becomes inactive after 60 seconds since the last activity.
Solus has recommended using Google Chrome as an alternative browser on iPad devices for Click & Collect.
Please note that the chime also does not happen if the patron starts the Click & Collect request, cancels it, and then starts it again. SirsiDynix is aware of this issue and there has been a JIRA assigned.