I found out that the MP3 media filter wasn´t working if there were 'new lines' between tags.
Ie not working:
<p align="center"><a href="../audios/2-1.mp3"><img
src="../images/cassette.gif" border="0" width="33" height="23"></a></p>
Ie working:
<p align="center"><a href="../audios/2-1.mp3"><img src="../images/cassette.gif" border="0" width="33" height="23"></a></p>
So, I modified the code in 'filter/mediaplugin/filter.php' adding an 's' in the line where the search string is defined:
$search = '/<a(.*?)href=\"([^<]+)\.mp3\"([^>]*)>(.*?)<\/a>/is';
That let the script treat the page text as a single line and resolve my problem.
I´m sorry if it has been discussed in other posts, I couldn´t find.
Someone tell me if should I send it to bugs track?