php and mime type

Hi list,

I'm hoping to find a way via php, preferably without using exec() or 
system(), to reliably identify an ogg vorbis file via php.

Using

$fi = new finfo(FILEINFO_MIME);
$mime_type = $fi->buffer(file_get_contents($upload));

results in 'application/ogg'

My site accepts vouchers for herpetology records, but only image and 
audio. I do not wish to support video vouchers, but if an Ogg Theora 
file is uploaded, it also produces the mime type 'application/ogg' when 
looked at with finfo class.

I need a way to distinguish ogg/vorbis from other types of ogg files, 
and it really needs to be w/o exec or system calls as they are currently 
disabled for security and I do not want to re-enable them.

Is there maybe a pcre extension or something designed to look at vorbis 
tags that could potentially be used to verify the file is ogg vorbis?

After upload I do transcode (from ogg to mp3 and mp3 to ogg so that 
html5 audio with flowplayer fallback works) by cron job that pulls new 
audio vouchers out of database, and I could (and currently do) check 
there, but all record submissions require at least one voucher and I 
would rather catch non ogg vorbis ogg files on submission before the web 
app creates the record in the first place.

Thanks for suggestions.