you could try leaving the mp3 preview fields empty for the PDF items and see what happens; i think the system might have a check to see if there is a preview specified;
otherwise you could use a default mp3 file with 5 seconds of silence, so making the mp3 preview non-functional for that particular item...
if you only need to do this for a small number of items, you could use ReReplacer (
http://www.nonumber.nl/extensions/rereplacer) and then copy the code that appears where you want to get rid of the audio previews and replace that code with some text like "no preview" or "PDF File"; In ReReplacer you would make a list of the things you need to replace in the first box, and then in the 2nd box the list of replacements; so you could do one ReReplacer entry for the whole album of poetry or whatever...
i use ReReplacer extensively in combination with Maian Media for all sorts of cool things, like formatting text in the tracks table, multi-line, tooltips, superscripts, and i also put placeholders in the language file, so that clients can edit their Maian Media language strings from the Joomla admin, using the ReReplacer interface...
-marc
