I have an English Tutorial website which contains sound files for visitors hear the correct pronunciation of words.
Dictionary.com have audio pronunciation as well, and when you click their little speaker icon, playback is instant without any obstacles like:

"Internet Explorer has blocked an Active X ...bla bla bla" or a superfluous dialog opens requesting user to verify intention to download... bla bla bla.
All of this will scare potential customers away.

I inspected the speaker graphic element on www.dictionary.com and Firebug showed me the following code:
PHP Code:
<embed id="speaker" width="17" align="texttop" height="15" flashvars="soundUrl=http://static.sfdict.com/dictstatic/dictionary/audio/luna/T04/T0442100.mp3" wmode="transparent" allowscriptaccess="sameDomain" salign="t" menu="false" loop="false" quality="high" src="http://static.sfdict.com/dictstatic/d/g/speaker.swf" type="application/x-shockwave-flash"
So, it appears that the answer is to just insert a Flash rollover triggering the mp3 right?

Given that over 90% of users have flash installed, I'm hoping that it would be a safe bet that flash is the way to go.

Any help appreciated.