Like NNTB said Konami are just too lazy and why should they bother when there is profitability in other formats (PS2 anyone?). Besides Konami aren't exactly in the best relationship with Nintendo.
Oh yeah NNTB i'm not sure if it'll help you but take a look
www.gbaemu.com and scroll down to check out the link for a Compressed Audio Playback System as the sites says. I don't know if it'll be of any use to you but the demos were of pretty good quality and were really small in size.