We discussed that some time ago here.
I brought up the old info about that only CoreGrafx and CoreGrafx II took in audio to it's standard DIN socket from the IFU, this turned up wrong, as when I tested it with an unmodded white PCE, it came CD Audio and ADPCM threw the RF, but all (White PCE, CG1, CG2) just do it mono as there is only one pin in on the EXT connector, so taking the audio from that socket modded or not should be avoided at all costs when running in the IFU, normal game sounds get stereo on CG1 and CG2, the White PCE with RF is everything in mono.
There might be a revision (White PCE) that does not input CD Audio or ADPCM on the EXT connector but that's pretty undocumented, actually we haven't look into what pin is inputing the audio at all.
So take the audio from the RCA on the IFU if you want stereo from all three audio sources.
Should be noted that the IFU is filled with components, and nearly all is for mixing audio.
As you are from PAL territory maybe this might be of interest.
http://www.retrosampling.se/Temp/JapanSpel/Bilder/Nec%20PC-Engine%20IFU-30%20(RGB%20Scart%20Mod%202).jpg