For reasons that are not entirely clear to me, lots of people still come here looking for Dreamcast code. This page should help you find what you are really looking for.
Dreamcast Linux is still alive and (sort of) well. If you install a Linux 2.4 kernel you will get a system that supports sound, X windows (if you really want that), VMU graphics and files. Work is ongoing on the 2.6 kernel - I've finally gotten round to writing a sound driver for 2.6 and it's even better than the 2.4 driver.
If you do want these things then the trick is to build and install your own Linux kernel (the pre-built one on m17n does not support sound or the VMU). Go to http://www.linuxdc.net to find out more. These days, though, the real place to look for help the linuxSH site.
If you are a Linux hacker and have just stumbled across this page, let me recommend the Dreamcast as an excellent platform on which to learn and use the wonders of cross compiling and embedded development. In an ideal world you'd have a rare, and consequently expensive, NIC (either the so-called "Broadband Adaptor" or the "LAN Adaptor"), but you can get by with a ten quid serial cable.
If you are interested in more general Dreamcast hacking, then you could do worse than try these places...