It plays fine here (8 Channels), it's your soundcard or more the setup of your soundcard, i had the same problem, but don't ask how i fixed it (i can hear even with the fix only the first 2 channels)
With the OpenAL output plugin, this STRM_BGM_MENU.brstm now works fine. However the wav file STRM_BGM_MENU.wav generated by the Test.exe r709 doesn't... that crashes Winamp outright after 1 to 2 seconds.
The directsound output plugin also can't play it, can't create the buffer for it.