Try using Chris’s Dynamic Compressor.
You can get it from here: http://pdf23ds.net/software/dynamic-compressor/
Find where it says “Download the plugin source” - right click on that link and “Save/Save As” the file “compress.ny”
Put that file in your Audacity plug-ins folder and restart Audacity. The effect will appear in the Effects menu.
You can usually leave the settings on the default values except for the first setting which is the compression amount - this usually needs to be set somewhere between 0.5 and 1.0 (higher values compress the dynamics more. Try setting it to about 0.8 - this will have a strong effect.