I have discovered how to amplify a whole stereo sound file. Is it possible to amplify only one channel (left, in my case)?
There’s several ways to do that. The way that I would recommend is to install and use the Channel Mixer plug-in (available here: http://wiki.audacityteam.org/wiki/Nyquist_Effect_Plug-ins#Channel_Mixer )
See here for how to install plug-ins: http://audacityteam.org/download/plugins