Alerts issued by the Reminders app are inaudible, or sound so low as to pass unnoticed, when I'm listening to the Music app with Apple ear buds or with external powered speakers attached to the Headset jack. I’ve tried different alert sounds to no avail. In contrast, these alerts sound loud and clear when I’m listening to a radio stream in Safari or in the Public Radio app.
Alerts issued by the Clock, Mail, Messages, and Calendar apps sound-off loud and clear in my ear buds or external speakers, and of course an incoming phone call interrupts Music play. All alerts sound just fine coming out of the iPhone's own speaker when nothing is plugged into the Headset jack.
One more thing: I’ve noticed that if I’m listening to the Music app at low volume Reminders alerts are noticeable. It appears that Reminders alerts, alone among all the alerts issued by iPhone apps, play at a fixed volume even though in Settings>Sounds>Ringer and Alerts>Change with Buttons is ON. So, if I crank headset volume up, Reminders alerts are drowned out.
I cannot recall if I had this problem before updating to iOS6 from iOS5. Anyone have any ideas about this?