muffled voice; indistinct voice
Describes a voice that is not clear, often because the speaker is mumbling, speaking with something in their mouth, or intentionally suppressing clarity.
He said something in a muffled voice, but I couldn't catch it.
Compound of 含み (fukumi, 'holding in, including') + 声 (koe, 'voice'), referring to a voice that is held back or not fully articulated.