During my smartphone lifetime, every iteration of iPhone and iOS I’ve owned has experienced a mysterious, life-altering bug: sometimes, and without any noticeable catalyst, my pre-programmed alarms will trigger without making any sort of accompanying sound. The visual alert will pop up, but thanks to the lack of sound, you won’t notice unless you’re already looking at the phone. For those of us that use our iPhone as our main nightstand fixture, having our only alarm forego making a peep — and thus, fail waking us up on time — is a huge problem. I’ve battled with the issue since the iPhone 4S, and every version of iOS since. There doesn’t seem to be a single solution, and no one seems to be aware of the cause.
The iPhone has had widespread issues with the alarm in the past, but that was generally related to daylight saving time. This issue seems to crop up every now and then, but it does not appear to be nearly as widespread as the DST problem. You can find traces of it throughout Apple’s support forums — either people inquiring about the exact issue, or at least something very similar. The proposed solutions are the usual ones: restart or reset your phone, wipe your phone and restore it from backup, or simply delete and remake your alarms. Of course, it’s always worth checking to see if the volume is working on the iPhone, but it always is — from text message bells and email alerts to games and music. None of the proposed solutions work.
The issue dates back pretty far — such as this inquiry from 2012, or this one from 2007 — and most (if not all) of the time, they go unanswered or unsolved.
Through somewhat tedious testing, I’ve found that what appears to cause the issue is unexplained interference from third-party apps. For example, if you leave a clock, podcast, or white noise app up and running on your phone, your alarm might trigger without making noise. It seems the only solution is to test your background and foreground apps one by one and see if any cause interference. If this, along with the usual reset and restore methods, don’t work — the only recourse is to take your phone to an Apple Store.