DLE and D2X-XL support putting level songs in a mission's hog file. Simply import them using the built in hog manager. To tell D2X-XL which song belongs to which level, you will also have to create a text file named <mission name>.sng containing one song per line. The song from the first line will be used for the first mission, etc. Empty lines mean that for the corresponding level the standard Descent midi song will be played. That's all. Simple, isn't it?