First, it helps to run the game from the command prompt if it exits early but cleanly, then you can see the error message, and can paste it here to ask questions.
Couple of possibilties: first remember that sounds are zero-indexed, meaning the first sound is 0. Also, if you're editing sounds.xml and have added a sound remember to add one to the numsounds. Also, you might want to run your files through XML Spy first to make sure they comply with the schemas.
If all this fails, send me the files in question and I'll have a look. I would need your ship file, your sounds.xml, and your .wav file.