The post you are referring to applies to older versions of mIRC. mIRC v7.38 now uses the more sophisticated TagLib library
instead, which can read a wide range of media files.
TagLib is fully unicode aware. According to the TagLib website, TagLib returns "the most appropriate bit rate for the file in kb/s. For constant bitrate formats this is simply the bitrate of the file. For variable bitrate formats this is either the average or nominal bitrate".
If the $mp3().bitrate value is not what you are expecting, can you provide a link to an example mp3 file where the wrong bitrate value is being returned and details such as what bitrate you are expecting?