If you want to do it automatically on the whole file, try Compressor or Steve’s simpler Level speech plugin. Please see here how to install Steve’s plugin: http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#install.
Or simply click and drag over one of the quiet parts and Effect > Amplify… . The default is to make the selection as loud as possible without distortion. Using this approach will sound very un-natural over the whole file.
Gale