Implement GD-ROM audio and status commands

[nkeynes]

Migrated from Mantis #17
Original Description:
"As used in the BIOS CD play screen. 40h appears to be status of some kind. Once this is done it should be fairly easy to add the audio functions (at least at a stub level).

Actually playing the cd audio from an image file needs to be implemented as well, probably via the main audio mixer. Native cd drivers can probably play directly."

[nkeynes]

Most of this was implemented a long time ago - audio playback from image files is still outstanding though