iPhone 4S. Sometimes no outgoing audio?

It looks like the ‘no outgoing audio on outgoing calls’ on the iPhone 4S is a glitch that will be fixed in the the 5.1 release.

Apparently it sometimes gets confused as to which audio output to use. Tapping ‘Speaker’ a couple of times when making a call has solved the problem on my iPhone 4S.

The Apple forum has a lengthy thread discussing this.