I was getting a blank space in my post instead of the player until after a bit of research and re-reading the documentation found a solution that fixed it for me in version 2.9 Below is what comes with the plugin's documentation:
IMPORTANT NOTE:
This plugin will only work if your theme allows inserting code in the HEAD and FOOTER sections of your blog. See [this page](http://wpaudioplayer.com/support/troubleshooting) for more details.
This is what I've done to get the player to show up in the posts:
Insert this in the header.php file:
<?php wp_head(); ?>
And this in the footer.php file
<?php wp_footer(); ?>
and it should work :-)
If your not sure how to write or locate the header and footer files, check wordpress codex for theme development help: http://codex.wordpress.org/Theme_Development