As far as I know there is no official soundtrack. Do you know about NSFs? They're like ROMS, but only the music information is there. You can even play them back at a higher quality than the original hardware was capable of producing. You can download the Cobra Command soundtrack in this format from many places, here's one:
Nintendo NSF Music - Zophar's Domain (
http://www.zophar.net/music/nsf.html)
Then you can play it back in something like this:
VirtuaNSF - Nintendo NSF Utility in Audio Players Category - Zophar's Domain (
http://www.zophar.net/utilities/nsf/virtuansf.html)