When I am praying, I usually become absent-minded. Does the Prostration of Forgetfulness (Sujood As-Sahw) make up for all kinds of mistakes committed during the prayer? How and when can we make it?
Also, is it true that cats can hear the voices of those who are being tormented in their graves and that this is why they are always frowning?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The Prostration of Forgetfulness has many rulings in the Sharee‘ah. The ruling to be applied depends on the type of mistake that occurs during the prayer.

Concerning your second question, it has been proven in the Sunnah that all creatures, except mankind and Jinn, hear the voices of those who are being tormented in their graves if they are close to these graves. In a Hadeeth on the authority of Anas the Prophet, sallallaahu ‘alayhi wa sallam, told us that when a disbeliever is tormented in his grave, he makes a sound that is heard by everything except mankind and Jinn. [Al-Bukhari and Muslim]

In another Hadeeth, on the authority of Umm Bishr the Prophet, sallallaahu ‘alayhi wa sallam, informed us that animals can hear the disbelievers being tormented in their graves. [Ahmad and At-Tabaraani] [Al-Albaani: Saheeh (Authentic)]

In a third Hadeeth, on the authority of ‘Abdullaah ibn Mas‘ood the Prophet, sallallaahu ‘alayhi wa sallam, confirmed that the dead are tormented in their graves and that the animals can hear the sound they make. [At-Tabaraani] [Al-Albaani: Saheeh (Authentic)]

Hence, the cat can hear the sound of those who are being tormented in nearby graves because it is an animal and is thus, included in the previously-mentioned Hadeeths. There is no proof in Sharee‘ah, however, that this is the reason why it frowns.

Allaah Knows best.