Possible Clues:

• [Grumble, grumble, grumble]
• Low talk from mummy in Germany
• Under-one's-breath sound
• Grumble in secret; mother (German)
• In a manner of speaking it's low for a parent in 5 down
• Speak quietly and indistinctly
• Mare, dead carp, grouse and beef
• Not say clearly
• That's a low way to speak about one's mother in Germany
• Speak or grumble in low clear voice