Convert High Res mp3s to Low Res Automatically on Server

I have an online music library which provides tracks for TV, advertising, etc. Music composers upload files to the site, we approve them, and then users can license these tracks for their video productions. When users pay for tracks, they can then download these 320kbps files from the site. What I need is for these 320 kbps files to be automatically converted to a low resolution file on the server, e.g. 32kbps, as composers upload them so that users can freely download them for previewing with their video productions. I have purchased processing capability with my host which I believe is necessary for this task. What I need the candidate for this job to do is to find an mp3 script that will convert all of the existing high resolution files that are currently on the server and then implement this to convert the existing files and then any new files as they are uploaded.