inaudible
Someone was muttering inaudibly in the background.
Almost inaudibly, he pleaded "not guilty" to the charges.
He started singing, almost inaudibly at first, then louder.

SMART Vocabulary: related words and phrases

Describing qualities of sound
audibly
boomy
chocolatey
chuff
clomp
crackling
ear-splitting
melodically
melodiously
metallically
murmurous
murmurously
silvery
squeaky
staccato
thready
tinnily
tunefully
tunefulness
tunelessly

You can also find related words, phrases, and synonyms in the topics:

Silence & quiet