Nestopia is an open-source NES emulator that tries to emulate the NES hardware as accurately as possible. It is available for Windows, macOS, Linux and the BSDs. There is a libretro port as well, see RetroArch for more information.
Install theAUR. Alternatively, install AUR for the development version.
No audio with libao backend
Currently the libao backend is broken. It is recommended to change the audio API to SDL in Emulator -> Configuration -> Audio .