godan verb (-su)
speak slowly and quietly
Describes a soft, unhurried manner of speaking, often with a gentle or subdued tone.
祖母はいつもほそぼそ話す。
My grandmother always speaks slowly and quietly.
彼は緊張してほそぼそ話した。
He spoke slowly and quietly because he was nervous.
ぼそぼそ is an adverb meaning 'mumbling' or 'speaking in a low, indistinct voice', often with a negative nuance of being hard to hear. ほそぼそ話す emphasizes a slow, quiet, and gentle manner of speaking.
小声で話す simply means 'to speak in a low voice', without the slowness or gentle quality implied by ほそぼそ話す.
The adverb ほそぼそ is a reduplicated form related to 細い (ほそい, 'thin, slender'), suggesting a thin, soft voice. Combined with 話す (はなす, 'to speak'), it describes speaking in a thin, quiet, and slow manner.