Glad tidings are for him who finds abundant invocations for forgiveness recorded in his book of deeds.
(Ibn Maaja)
When imam says Allahu-akbar when going in sijda, ruku etc. do we say it softly as well?
Yes, we should say it softly. Some people say it loudly, this is bidat. It is against the Quranic instruction;
“وَقُومُوا لِله قَانِتِينَ”.
The sahabi says
“فأمرنا بالسكوت”.