Setting aside graphics for a moment, which do you feel was the last big generational leap in audio for consoles, and how so?

The transition from cartridge sound to CD sound in supported devices in the late fourth generation and breaking through in the big time (PlayStation) in the fifth generation was the biggest leap. And 4-channel surround sound became way more common in the fifth generation than it was on SNES and CD games in the fourth generation.
And even if you only had TV speakers or a stereo setup, the leap from SNES to PS1 is significant and obvious. Admittedly, most PS1 and early CD games didn't have live orchestras yet for the music because of storage limitations.

Which was the last big one though, like do you feel we've had a big leap since the 4th to 5th transition?

The shift to CD-Rom during the 16-bit generation marked the last big one, IMO. Red book audio became a factor. It hasn't really moved much since then.

I would say 5th to 6th had big moments with 5.1 Surround introduced and more orchestration than the 5th Generation had.

7.1 Surround Sound got introduced in the 7th Generation. Dolby Atmos was introduced in the 8th Generation. 

Some big leaps, but I stand by my choice. 

Fourth to fifth. When it comes to audio, nothing compares to going from carts to CDs. That is the last big leap.

And a lot of other bigs are going to be between the third and fifth, as well, because that's when gaming was at its peak as far as creativity and innovation. They were paving the road then, making these amazing things up out of thin air, and there were no expectations. Well, there was, but the guys and girls making the consoles and games were exceeding them to such a degree, that us gamers were incapable of sometimes believing them. DKC, for instance, my goodness. I can't begin to properly explain what sort of mind **** that was. And then later with games like MGS or OOT. Everything from NES to PS2 is locked in for GOAT status because of it. Gamers today simply will never be able to experience that level of discovery and novelty. It was drenched in dopamine. 

4th to 5th. Tho I need to put an * on that as 4th had Sega CD and PC Engine CD which had CD sound. Last big one? I guess 5th to 6th when more real instruments were used and orchestras. Dolby Logic and Surround sound.