Steve made a plug-in for Audacity called “LevelSpeech.ny” for that purpose … Improved "leveling" for speech - #16 by steve
Alternatively try Steve’s Limiter plug-in … Missing features - Audacity Support
[ see here for how to install plugins into Audacity … Missing features - Audacity Support ]
When using either of these plugins the quiet-bits will be made louder , but inevitably, they will be noisier [hissier] than the loud-bits.