There is no Winamp plugin (or any other app) which can decode the ATRAC3-encoded .bgw files yet, as far as I know. There are plenty of apps out there that can play back ATRAC3 audio, but the problem is that the audio is encapsulated in Square's in-house (I assume), proprietary .bgw/.spw container format. The only programs that can read that container format are the winamp/foobar plugin and polutils, and none of them know what to do when they run into audio data that isn't PCM.