Hmm not sure if it will work, but please give this a try first.
In your newattachmentbit template, find:
Code:
<b>[MP3]http://www.yoursite.com/vb/attachment.php?attachmentid=$attach[attachmentid].mp3[/MP3]</b>
Replace that by:
Code:
<b>[MP3]http://www.yoursite.com/vb/attachment.php?attachmentid=$attach[attachmentid].mp3[noguest]This text is only for registered memberp[/noguest][/MP3]</b>
That might do the trick, if you have installed the [noguest] example that is.
To make it work your vBulletin setting for "BB Code Parser Function" should be on "bbcodeparse2_recurse()" or "bbcodeparse2_regexrecurse()" (default)
Alternative 2:
just add the [noguest]text[/noguest] to the end of the custom bbcode MP3, just after the line "</object> "
Both not tested by me.
If it won't work, i will dig a bit more.