Give :00, :15, :30 and :45 each their own sound.
Set the interval to every 15 or 30 minutes, and each mark gets its own sound. Four quarters, four distinct sounds if you want them, so you can tell which quarter of the hour it is without glancing at a screen.
Don't want a chime at :15 or :45? Set that mark to No sound and it's skipped completely: not muted, not shown as a silent notification, simply never scheduled. The :00 mark always keeps a sound, since an hourly schedule would otherwise have nothing left to fire.
A full week of quarter-hourly chimes across every selected hour can ask for more than the 64 pending notifications iOS allows any app (Apple's UserNotifications documentation). Tinny(not) spends that budget on coverage instead. The top of each hour first, then :30, then :15 and :45, with days taking turns, so whole days don't go quiet. If chimes have gone quiet for a different reason, the troubleshooting guide covers the rest: the ring switch, Focus, per-app sound, and permissions.
Yes, once the interval is every 15 minutes. Four marks, four sounds, and you can tell the quarter by ear.
Set a mark to No sound and it's never scheduled at all. The :00 mark always keeps one.
iOS lets each app keep at most 64 pending notifications, and a week of quarter-hour chimes can need more than that. Tinny(not) puts the top of each hour first and rotates the rest across days, so no whole day falls silent.
Tinny(not) is free, hourly chimes included. Quarter-hour intervals are one of the extras in an optional one-time purchase.
Get Tinny(not) on the App Store