The Briefing Room theme, or Ambience theme is the theme that plays while all the hunters (Samus, Rundas, Ghor, and Gandrayda) are being briefed in the Ready Room. The same theme is also used as a filler theme when there is no specific theme for a given time or area--hence the "Ambience theme" name.
You can listen to Briefing Room/Ambience Theme (Metroid Prime 3: Corruption) here.