Harrumph – I have already mentioned that I tried running a separate 3.5mm to 3.5mm audio cable from the MD to the 3.5mm AV1 In jack. This is a completely separate cable from the RetroGaming scart cable. There is still the same distortion.
I have tried headphones from the MD headphone jack and there is no distortion.
Only time distortion is present is when audio (either via separate 3.5 jack cable or via RetroGaming scart/audio cable) is running through the OSSC.
Has anyone else tested this using the games affected above?