All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The scholars considered raising the voice when addressing the parents as undutifulness to them and ill-treatment, in the same manner that they considered lowering the voice when speaking to them as an act of kindness and benevolence.
Ibn Hajar Al-Haythami said in Az-Zawaajir:
“The grave major sin number 302: “Undutifulness to parents or one of them ..... the child should not respond to them in a harsh way, look at them in a stern manner, or raise his voice on them. Rather, one should be with them like a slave in front of his master....” [End of quote]
Mafaateeh Al-Ghayb reads: “You should know that being kind to the parents is to serve them, not raise your voice on them, and not to speak to them in a stern manner, ...” [End of quote]
The Fatwa of the Standing Committee was asked about whoever raises his voice when speaking to his mother, and it answered as follows: "You did an evil thing by raising your voice on your mother..." [End of quote]
This is the principle ruling on raising the voice on the parents, but it appears –Allah knows best – that if there is a need, such as making them hear, because of them having hearing difficulties (or because of being far away), then there is no harm in this and it is not considered as undutifulness to them, especially if this is the only way to make them hear when addressing them.
Allah knows best.