Fetching details from the dictionary...
in a subdued voice; in undertones; in a whisper; in a mumble; in a murmur
dry and tasteless; stale
彼かれはぼそぼそと話すはなす。
He speaks in a mumble.
何なんかぼそぼそ言っいっている。
He's mumbling something.
返事へんじをぼそぼそと返しかえした。
I mumbled my reply.