In the new version of Moodle (1.9.4) the unwanted text (MP3 audio) is shown when the mp3-player is used.
This happens for example in a test where questions with audio.
I think that the problem is in the file filter\mediaplugin\filter.php
This is the only file in this folder that changed compared to an earlier version of Moodle.
The problem was fixed by copying the entire filter\mediaplugin folder from an older version to the newer version so I'm not 100% sure but the problem is in this folder,
See the attachment for a picture of this error (it's a question in a Dutch test)