JBM: Match loudness for stereo and mono in ParamUpmix mode

Basic info

• Commit SHA: 569c63df
• Platform: all

Bug description

ParamUpmix which operates at 7.1.4 160kbps when rendered to stereo and mono doesn't quite match output levels with other modes in the JBM path. Related to #787

Ways to reproduce

(Clear steps or refer to a failing automated test, e.g. with a pipeline link)