If Normalizing the entire file gives you an acceptable result, create a Chain containing Normalize and an ExportWAV command then apply the Chain to files that are in the same folder.
If Normalize does not make the files equally loud, add the ReplayGain Nyquist plugin to Audacity’s Plug-Ins folder. Then assuming you are using the latest Audacity 2.1.2, open Effect > Add / Remove Plug-ins, select ReplayGain, enable it, then OK. Then add ReplayGain instead of Normalize to the Chain.