Well, if you are willing to use a plugin like Quicktime, you could control it using Javascript like so.
<embed src="success.wav" autostart=false width=512 height=32 id="sound1"
enablejavascript="true">
<script>
document.getElementById("sound1").SetRate(0.5);
</script>
To my knowledge, RealPlayer and Windows Media Player do not have a similar option. F̶o̶r̶ ̶V̶L̶C̶ ̶M̶e̶d̶i̶a̶ ̶P̶l̶a̶y̶e̶r̶ ̶y̶o̶u̶ ̶m̶i̶g̶h̶t̶ ̶b̶e̶ ̶a̶b̶l̶e̶ ̶t̶o̶ ̶u̶s̶e̶ ̶̶v̶l̶c̶.̶i̶n̶p̶u̶t̶.̶r̶a̶t̶e̶
̶ nevermind, it is a readonly propety. (see the doc https://wiki.videolan.org/Documentation:WebPlugin/ )
I'm not going to get into details with Flash, but Flowplayer seems to be able to play in slow motion http://flash.flowplayer.org/plugins/streaming/slowmotion.html and can also play MP3s apparently http://flash.flowplayer.org/plugins/streaming/audio.html
However, as for the HTML5 audio tag, there is the playbackRate Javascript property, as seen here on w3schools (couldn't find it anywhere else ^^' )
http://www.w3schools.com/tags/av_prop_playbackrate.asp
mySnd=document.getElementById("audio1");
mySnd.playbackRate=0.5;