It's been a long time since I looked at 'onload' audio. Ewonline is right in that Flash offers the best cross-platform player. The best solution I can see at the moment is
XSPF Playerhttp://musicplayer.sourceforge.net/
It uses Flash, plus an XML playlist to play MP3 file(s). It needs the Flash plugin
version 7 or later, which most modern browsers should have (it plays in 'Epiphany' on Linux, so what more could you want!).
You can set it up to play an MP3 automatically on page load.
You can split your audio into several MP3 files and it will play them sequentially.
You can stick it in a XOOPS block.
Users can switch the audio off, or select files if you use the 'extended' version.
In the code below, replace 'example.com' with your website URL.
1) Download the 'slim' version 0.2.3.
2) Rename the folder 'xspf'.
3) Place the xspf folder in your XOOPS root.
4) Add MP3 file(s) to the folder (in this example 'intro1.mp3' and 'intro2.mp3').
5) Create an XSPF format playlist in a plain text editor, e.g.
<?xml version="1.0" encoding="UTF-8"?>
<playlist version="0" xmlns = "http://xspf.org/ns/0/">
<trackList>
<track>
<location>http://www.example.com/xspf/intro1.mp3</location>
<annotation>Text to be displayed for first file</annotation>
</track>
<track>
<location>http://example.com/xspf/intro2.mp3</location>
<annotation>Text to be displayed for second file</annotation>
</track>
</trackList>
</playlist>
Save this as 'playlist.xspf' in the xspf folder.
Create a new block in XOOPS (HTML) and add the code:
<object type="application/x-shockwave-flash" width="100%" height="170"
data="http://www.example.com/xspf/xspf_player_slim.swf?playlist_url=http://www.example.com/xspf/playlist.xspf&autoplay=true">
<param name="movie"
value="http://www.example.com/xspf/xspf_player_slim.swf?playlist_url=http://www.example.com/xspf/playlist.xspf&autoplay=true" />
</object>
Remember to make the block visible to the anonymous users group. Omit the 'autoplay' param if you don't want the audio to play when the page loads.
HTH
Tip: watch your 'x', 'f', 'p's and 's's when defining paths. It's a bit of a tongue twister.

This works in IE6 (now). However, Microsoft have done something really bizarre with later Internet Explorer upgrades which may stop any of this working.
Some info
here
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