I'll quote a few references I've found that seem to be part of the changelog. Unfortunately, the lastest version of mIRC does not include the full change log, but I've found a few places that at least go back to v6.0.
As of mIRC v6.0:
80.Added WMA/OGG support to sounds dialog, /splay and /sound
commands, and added new identifiers.
$sound(filename|type)
Returns information about the specified file if a filename
was specified, or the associated folder for a filename or
file type, where type can be wave, midi, mp3, wma, ogg.
When used with an mp3 files it supports the same properties
as $mp3().
Note: this replaces $wavedir, $mididir, $mp3dir, and $mp3().
These are still supported for now but no longer documented.
$insong
Returns information about the mp3, wma, or ogg file that
is being played, same properties as $inmp3.
Note: this replaces $inmp3, which is still supported for
now but no longer documented.
Do you recall such an entry in the change log by any chance?