To start, let me just say I am using the TV Speaker for all of this. In all other respects this is perfectly adequate for my setup. I intend to get some dedicated sound system in the future but this may not solve the untelying issue.
When using Game Mode, the sound mode of the tv is locked to 'Game Mode'. This is understandable, however, it appears that this audio mode does not simply leaving the audio alone as you would expect (the point of game mode being to bypass processing in order to decrease input lag). Instead Game Mode audio appears to be running some sort of signal processing and altering the sound. The problem with this is that the whatever it's doing makes some things sound just awful. This is particularly apparent when voice and ambient sound effects overlap (such as the commentary over the crowd noise in FIFA for example). It appears the audio is selectively amplified to bring out voices, however this results in some serious artefacts as the amplification results in the ambient audio also fluctuating wildly around the "edges" of the vocals. It is almost as if there Is severe compression and/or ducking occuring but with too long a release time. I have tested the other sound modes (with Game Mode turned off) and it seems that the "Adaptive Sound" does a very similar thing. I suspect that Game Mode audio is very similar to Adaptive Sound.
If this is the case then there should absolutely be an option to change the sound mode to Standard (or a completely unprocessed mode) while using Game Mode in any future firmware update.
I may of course be wrong and Game Mode audio may not be adding processing but I have tested the situations in which I noticed the audio issues on other systems and these issues don't occur.
If anyone knows anything more about Game Mode audio and wants to weigh in I'd be really interested to learn.
Yeah it's amazing how once you hear it you can't focus on anything else. Can't focus on Martin Tyler's scathing comments and forced pronounciation of "De Bruyne"! Unfortunately not a proper "fix", I have now started using a Yamaha reciever (RX-V673 ) for all the audio (consoles into reciever, HDMI ARC so the reciever handles any audio from the tv (smart apps and such)). While this has sorted the audio for me, I dont consider this a proper fix as basically it means just 'spend some more money' and offload the audio to another device (in my case I was extremely lucky that a family member was upgrading their system and the Yamaha was up for grabs). It should be possible to disable whatever processing is causing this really. Sorry I cant be of more use. It's definitely an issue that needs to be addressed.