A good and easy to use compressor is “Chris’s Dynamic Compressor” http://theaudacitytopodcast.com/chriss-dynamic-compressor-plugin-for-audacity/ You will probably want to increase the “Ratio” setting to about 0.8.
Or there is an “Automatic Gain Control” type effect here: https://forum.audacityteam.org/t/agc-automatic-gain-control/26706/1
Or there is the built in Compressor effect http://manual.audacityteam.org/o/man/compressor.html