I am assuming you are asking about the mp3 file that is on the same page as the flac audio file up at the Internet Archive since that is the only version of my audio I can find that I would describe as a zipped mp3 file. The short answer is that I do not explicitly zip that mp3 file. I upload a flac audio file, which is just a losslessly compressed raw audio file. The Archive automatically transcodes that to other formats, including apparently a zipped, variable bit rate mp3 file, as well as Ogg Vorbis. If you look more closely at that page, there is also a link to a non-zipped version of the VBR mp3 they encode from the flac file.
I don’t use the Archive primarily to serve files, more to have an off site archive of the finished raw audio. I include the link to the Archive page for anyone remixing my work under the Creative Commons license terms so they can download the raw audio if they want rather than remixing a crummy lossy encoded file.
Please note that on my site, there is a link to the MP3 file that I encode, which is neither zipped nor variable bit rate (the latter causes problems with some players). This is the exact same file that is enclosed in the mp3 feed. It is also the same file that the embedded Flash widget plays in each podcast post on the site itself. The files I manually encode–in AAC, MP3 and Ogg Vorbis format each served through a dedicated feed–all have full metadata as well as being encoded at the bit rates I prefer for best audio quality for each particular format.
Since the additional, automatically encoded files at the Archive seem to be causing some confusion, I’ll look into the possibility of turning off the automatic encoding.