using ov_test_callback with function CALLBACKS_STREAMONLY will cause
scanning to stop after the comment field.  ov_open (and ov_test)
default to CALLBACKS_DEFAULT which scans the file structure causing a
huge slowdown.  The speed improvement is huge: It scanned my files
around 10x faster This procedure has been recommended by monthy (main
vorbis developer) and was said to be safe for scanning files.