Switching left and right sound channels

[John Oxley]

Hi,

I am running FreeBSD 5.4-RELEASE-p2.  I have a CMI sound card, 
04:01.0 Multimedia audio controller: C-Media Electronics Inc CM8738 (rev
10)

I think either my headphones are dodgy, or the sound card is
misconfigured, and sound is coming out the wrong speakers.  How do I
swap the channels in software.

TIA,

John