With the new FLAC, a new tag was invented to store the ReplayGain reference level.  It maps to the vorbis comment field name "REPLAYGAIN_REFERENCE_LOUDNESS".  I propose adding support for this tag alongside the other ReplayGain tags.  The purpose of the tag is to put an end to the ambiguity that is inherent to the relative gain adjustment values.  So in a sense, the invention of this tag comes 6 years too late, but hey!

I'm attaching a patch to add the tag as GST_TAG_REFERENCE_LEVEL.  I oppose taking over the "reference loudness" term used for the comment field name because doing so makes no sense; the ReplayGain proposed standard always uses the term "reference level" to refer to the 89 dB (back then 83 dB) target output level of the reference waveform.  The term "reference loudness" is only used once (in the MATLAB script), where it refers to the result of the RG function (which is meaningless by itself).

Another patch against gst-plugins-base will be attached that adds the new tag to gstvorbistag.c, which should enable support in all formats that use vorbis comments.

As part of another bug, I'll add support for the tag to the element(s) that are part of the replaygain plugin.