I also use DFX Audio enhancer for enhancing voice quality in my Windows Media Player. The problem with DFX is that it’s not free and I love freewares.
While crawling the internet, I came across a very good site which offers to make the audio louder. It has four levels of loudness. Light, Loud, Louder, Loudest. Choose whichever you want to make your audio.
No software installation is required. You just have to upload the audio clip and then download the louder version instantaneously. The limitations of this service include that it only allows 10MB of audio clip to be processed. And the other limitation is the file format. Only wav and mp3 formats can be processed. But usually only these file types have a low audio quality. So it normally remains good and useful most of the time.