Does the 30 minute file just cut off abruptly at the end?
Are you using Mixcraft 6 Home Studio version 6.1 build 216 or 217? If not, choose "Check For Update" from the Help menu in Mixcraft 6 HS.
Mixcraft 6 Home Studio costs less than other Mixcraft packages and the option to mix down to MP3 is one of the reasons. Here's chart that shows what's included with the various Mixcraft packages: Mixcraft 6 comparison chart