Though GX::Transcoder is for free, it is more powerful and flexible than other conversors which you have to pay for.
It allows you to adjust the velocity and the quality of the extract and convert processes. It is very easy-to-use, you can drag&drop files to convert them with just two clicks. However, if you are an expert you will not miss any feature, because it is very complete.
Besides the audio format conversion features, yo can also use GX::Transcoder as an audio player, because it is not far away from other ones.
Thanks to its very clear interface, you will be able to watch the full process in real time.