Gol D. Roger was the most prominent wielder of the “Voice of All Things” power and made great use of it. Roger was able to locate the Poneglyphs by hearing their shouts.
Shanks chose Luffy because he reminded him of Roger. This is further seen by the parallel of Roger revealing his dream to Oden and Whitebeard. Ace and Sabo responded the same way when Luffy revealed his dream.
Also, Shanks mentions that Luffy said the same thing that Roger did to Rayleigh which is probably what was said in that scene.
Shanks took a chance on the future by speculating that Luffy would also have the ability to hear All Things. Thus, Luffy should be able to hear the Poneglyphs, including the last Road Poneglyph whose location is currently unknown. I think that the last Road Poneglyph may be somewhere that’s unmovable which is why Shanks doesn’t have it. Thus, the only way to reach the last Road Poneglyph is to listen for it.
What if the last Road Poneglyph is in the Florian Triangle – a lot of ships go missing there and all sunlight is blocked. This would be the perfect place to put a Poneglyph because only someone who can listen for it would be able to find it. I think there is an island in the Florian Triangle that is being protected by those big creatures that we got no explanation for back in Thriller Bark.
The people who go missing or the empty ships may be a result of these creatures protecting the island which has the last Road Poneglyph. Luffy will go to that island using the “Voice of All Things” and that’s how he’ll find the last Road Poneglyph.
*Theory by ArrowsUp