I have the same issue. Worked fine back in November. With the latest files it does not work.
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Design Studio SDK Extension Sample Audio
Bundle-SymbolicName: com.sap.sample.audio;singleton:=true
Bundle-Version: 1.0
Require-Bundle: com.sap.ip.bi.zen.rt.components.sdk.eclipse
Bundle-Vendor: SAP
i have added the xsd file in the correct location.
I have created several new workspaces
imported files serveral times
installed latest eclipse
redownload the files from SAP