Is it better to say the Tasbeeh and Du'a silently or audibly during Rukoo' and Sujood, and which is preferable?

The prescribed way for glorification (tasbih) and supplication (du'a) during bowing (ruku') and prostration (sujud) is to say them silently. This is particularly emphasized if vocalizing them would disturb other worshipers. However, there is no harm in vocalizing them softly sometimes.