RetroPie (or libretro) does not have a ZSNES core; ZSNES, apart from being wildly inaccurate, is largely written in x86 assembly and so there is no ARM version.
Additionally, the more modern snes9x cores have essentially the same sound emulation as higan (blargg's SNES_SPC library). I believe higan has done some modification of that code since importing it but regardless, the sound emulation in snes9x is very, very accurate.
Higan is indeed unparalleled for those who truly care about extreme accuracy, but snes9x is 99% there for the vast majority of users who just want to play games.
Additionally, the more modern snes9x cores have essentially the same sound emulation as higan (blargg's SNES_SPC library). I believe higan has done some modification of that code since importing it but regardless, the sound emulation in snes9x is very, very accurate.
Higan is indeed unparalleled for those who truly care about extreme accuracy, but snes9x is 99% there for the vast majority of users who just want to play games.