Abel obtained witness that he was righteous; God somehow revealed to Abel that he was accepted by him, the living God, and that he had pleased God. Perhaps God sent fire down upon his sacrifice; we are not told the details.
Hebrews distinguishes between the two offerings by saying that Abel had faith and Cain did not. The presence of faith makes all the difference and that is the subject under consideration. Without faith no worship is acceptable to God, and we can only approach God in a spirit of self-righteousness. Faith accepts the fact that we are sinners and sees the need for atonement. So if Cain failed to first approach God with a blood sacrifice, he not only lacked the faith that is required in any approach to God, but he also brought the wrong sacrifice. Faith would have taught him the need of atonement.
May your faith in God, your saving faith that you trusted in Christ for salvation, your ongoing faith that leads you to speak to others, and to pray for others; may this enable you to speak even after you are dead. You speak to other souls and if God uses your words and somebody is saved, then they will speak to others, and there may be a chain of blessing that will go on for generations.
Think of Joseph Passmore and James Alabaster. They were Spurgeon's first publishers. Joseph Passmore went to the very young Spurgeon and he said he had a proposal: to publish one of the Sunday sermons and have it on the streets on Monday or Tuesday at the latest as a kind of penny sermon. He would provide the stenographer to take down the shorthand, and set it all up in print. He proposed that to a very surprised Spurgeon and it became the publishing project of a lifetime. After his death the publishing venture passed into other hands. Now of course it is on the Internet and perhaps people haven't heard of Passmore or Alabaster, but their voices also speak long after their death. All of us who love Christ can speak long after our deaths. It is the extraordinary thing about Christian testimony: you testify yourself; you support the testimony, the work of the church; you engage in corporate witness and of course you intercede, and it can be said of you, he or she ‘being dead yet speaketh’.