Had this problem today. Got a text message but there was no audible alert even though my phone wasn’t muted.

When I played with the volume the overlay said “ringer (headphones)”. It thought I had headphones plugged in. Uh oh.

After some Googling, I found there are a couple possible remedies to this that can work. I’ll start with the one that worked for me…

It’s possible for dust, lint, etc. to get in the headphones port and make the phone think headphones are plugged in.

I took a can of compressed air with the skinny straw attachment, stuck it in the port and blew it out with air for a few seconds. Poof! Problem solved. Phew!

The other methods that people have reported working:

1. Plugging and unplugging headphones until it starts working (maybe just another method of clearing debris?)

2. Start up a song in iPod with the headphones plugged in, then shut down iPod and return to desktop with the headphones plugged in, then unplug the headphones.