Essentially, to take a Sony Ericsson (often anything that isnt a W-Series, e.g. K770, C702, S500) and cross-flash audio drivers from a Sony Ericsson W-Series. Often to unlock extra features such as Mega Bass, Clear Audio and to enhance audio quality. Done on Sony Ericsson feature phones that run on the same platform as the System-on-Chip is not much of an issue.
I just Walkmanized my Sony Ericsson C902 by installing the audio drivers from the Sony Ericsson W980. Now I've got a James Bond phone that can also act as a dedicated MP3 player with a Cyber-shot camera.