The format isn't supported, it seems, so I gave the extracted audio the extension "DIDSP". If someone wants to take a look at the format (since I can't seem to compile in_cube), here are a few extracted examples:
Oh, and feel free to mirror the IDSP extractor on hcs64.com. It won't work on any game other than Defender, however. If there is a need for a real IDSP extractor, i'll write one.