Possible Clues:

• Whisper; complain
• Soft voices (of discontent)
• Indistinct mutter
• Speak in a low voice, whisper
• There's some low talk of there being a couple of drinks back here
• Two drinks back, and there's little you'd get from it, by the sound of it
• Quietly grumble, rejecting drink twice
• Low continuous sound
• Funny (peculiar) echo, as of babble
• Grumbling sound