[Skelsgard]

Try muxing the fixed mono WAVs into a interleaved 6-ch WAV with Wavewizard, just make sure that the files are in WAV order from top to bottom:
FL
FR
FC
LFE
SL
SR

In wavewizard, Edit --> Preferences. Enable Stream manipulation: Merge files, Ignore size in headers: always, Output format: Wave PCM.

Then you feed this 6ch WAV to Aften (aftengui).