Hi, this is a strange side effect of the maven release process. For JMX 0.3, and for 0.3 of other top level modules, a zip of all the files from SVN is produced. In this case it is jmx-0.3-source-release.zip. Each of the sub-modules of JMX have the following released artifacts: binary jar, source jar, sometimes a javadoc jar, pom. There is an asc, md5 and a sha1 file each of them. There is an md5 and sha1 for each of the asc files. The one you have picked: org.apache.aries.jmx-0.3-sources.jar is in fact the sources jar for the top level module.
There is no java source in the top level module so it is effectively empty. If you need the source for the entire module (including sub-modules) pick the source-release.zip file. If you need source for just one of the sub-modules, pick its sources.jar.
If this doesn't fit with your usage pattern (e.g. in your IDE) please start a discussion thread on the dev@aries list and we can look at it. Thanks.