So many people have problems because they load the wrong firmware and lose sound, have you considered naming the main one the version number “FWxx” and the non-audio one, “FWxx OSSC 1.5 and older”. I know it’s clear to the regulars here, but it seems to happen a lot!
It would be inaccurate to describe the non-audio builds as being specific to the 1.5 and earlier hardware revisisons, because they are also compatible with the 1.5 with audio addon and the 1.6. I would also expect the audio firmwares to work on the 1.5 hardware, but that would be pointless.
Either way, it looks like the firmware repository (linked at the link above) has been restructured so that the audio-enabled firmwares are in a “release_audio” directory and the non-audio firmwares are in a “release_non-audio” directory, which should eliminate confusion.