ANSWER #1: ZIP is NOT a controlled format. It is public domain. Thus, there is no "Standards" on it. MP3G Toolz has done their own thing, Tricerasoft also, and use something called "MAX" compression. Most programs, Win Media Player included, can't play these files.
One of the features we programmed into MTU's Songerverter is to AUTOMATICALLY change a ZIP with MAX compression to a ZIP with Standard compression. After that processing, most .zip files will work.
ANSWER #2: The Unzip code we use in Microstudio 4 is one of the best available. However, the version release we built into 4.005 was still very intolerant of ANY FORM of errors in the ZIP HEADER. Thus, when you unzipped, you removed the zip header, and any error(s). The fact the songs played when unzipped tells me the zip had errors.
In Microstudio 4.006, soon to release, we are building with the latest version of the Unzip code. We have tested it with ZIP files with errors, and it is much more tolerant. We think a lot of the problem ZIP files will now work in Microstudio 4.006.
WARNING!
You folks have no idea how bad ZIP files can be. Many of you download ZIP files over the Internet. You have no clue what they contain, or what program was used to ZIP them. They could include Viruses, Trojan Horses, Keystroke Loggers, and any manner of malware zipped within them and your identity could be compromised with a single ZIP file.
We have built into Microstudio and Hoster, somewhere around 15 error traps in our ZIP processor to detect and correct common errors found in ZIP files from customers who send them to us.
ZIP files are a true pain in the rear!
I just wanted you all to realize how complex it is to "play" and process ZIP files. 
Anything... anything else is superior to ZIPs. PERIOD!