Faith here is loyalty to an unseen state until it appears. Abel's better offering is the gift of a heart that assumes the wish already fulfilled, while Cain's is the labored substitute trusting effort over assumption. The witness that he was righteous is the inner confirmation received once a person has fully believed, before anything outward moves. God testifies of the gift by making the assumed state feel already accomplished.
"Being dead yet speaketh" means an imaginal act, once made, goes on working long after the moment that made it. All things already exist; imagination only rearranges the unseen until it shows itself. Faith is not sight. It is staying faithful to a wished-for state when nothing yet supports it. The person who assumes now that he is what he wants to be has made the offering. That inner act keeps speaking until the world says the same thing back.