When embedding sound clips in a web page, I've found that mplayerplug-in plays sounds twice if they're less than 2 seconds long. I've included a test case where the original sound plays twice, as does the same sound with silence added up to 1.9 seconds, but the same sound with silence padded up to 2.1 seconds only plays once.
The same test behaves properly on Firefox on a Mac (which plays sounds using QuickTime), so I'm pretty sure the problem is with mplayerplug-in.
I'm using mplayerplug-in 3.40, and my Iceweasel version information is:
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:18.104.22.168) Gecko/20070310 Iceweasel/22.214.171.124 (Debian-126.96.36.199-2)