JBM: Match loudness for stereo and mono in ParamUpmix mode
### Basic info
- Commit SHA: 569c63df9263b4bb6c690d56dece391eb0c53b95
- 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)
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issue