A number of games use mixed audio/data CDs with the audio portion containing the background music as (AFAIK) standard red book audio CD tracks.
When one of these discs is inserted, OS X mounts the data and audio parts separately. The game importing process in its current form only handles the data part.
If the original CD is in the drive when playing the game (auto-mounted inside Boxer as drive E), the CD audio plays just fine. Of course, the optimum way would be to have the ability to play the audio without having the CD in the drive, meaning Boxer's import process should also import the audio tracks, preferably compressed in some suitable format (MP3, AAC, OGG or whatever is convenient and not likely to cause software patent troubles; FLAC or Apple Lossless if avoiding lossiness is a must, though I doubt it with modern transparent lossy encoders).
The intuitive way seems to be to show a prompt when the data part of the CD is dragged to the import window, asking whether to also import the audio tracks into the game bundle or not, probably defaulting to "Yes".
How Boxer, when given OS X's data mount of the CD through drag & drop, should find out if the same disc has a valid audio part, is left as an exercise to the coder :-)