The envelope tool would be one of the best approaches, http://manual.audacityteam.org/o/man/envelope_tool.html
but that could be very tedious if it is more than a very short recording.
An alternative method that you could try is to apply an “Automatic Gain Control” effect to the recording (try it on a short section first to get the settings right (Ctrl+Z to Undo). The plug-in and instructions are here: https://forum.audacityteam.org/t/agc-automatic-gain-control/26706/1