Hi
You don't say if this is after export or during preview..Anyway...
Presuming you have saved your project..If not then "Save Portable Project As" (where all your files are saved into folder of your choosing along with the,vpj file)
Open VP and navigate to this folder and then load one of the problematic music/song files. Drop it onto the audio track and check that it loads and converts completely and has the expected duration on the timeline and then play it. Does it play correctly?
If it looks and behaves correctly then close VP, reopen it and load the project (,vpj) file saved above. Now check the same music file on the audio track. Is it all there and does it play correctly when you preview your video?
Nat