What reports can I use to answer Traceability questions related to builds?

When you say "source code modules modified within the build", do you mean modules that have new versions checked in to them as part of the build, or do you mean modules that have been modified since the last build?   Assuming it is the latter, when you build from the RTC SCM system, for a team build, it does list the work items that contain change sets that have been added since the last successful build.   If you want to compare the source files of this build against some other configuration, you can compare the snapshot created for the build with another snapshot/stream/workspace which contains the configuration with which you want to compare it.