My ESS1688 experience was always some kind of halved-sample-rate impression under DOS overall, much less about a filter/lowpass. but that was ages ago and that was only under DOS and not in stuff running on Windows 95 etc (the sample rate was fine in windows games/apps).

It would be a shame if the full potential of the card is locked behind a Windows wall.

EDIT: I would like to add that it could be that the sounds are more of lower sampling quality rather than filtered. I am an amateur and cannot differentiate it properly yet but I will recognize that something sounds wrong. Another mystery to me is that ADPCM sounds (i.e. in Duke Nukem II) don't work even though they work with ES688. It could be correct behavior as I couldn't find any mention of ADPCM support in ES1688 data sheet (only ESPCM compression is mentioned) however it's weird that one chip supports it and other doesn't especially since both chips should be very similar apart from OPL3 support.

The ES1688’s I’ve tried sound great in DOS and Windows but I have noticed that using the native support for the chipset that some games provide can result in super filtered digital voice - Warcraft II for example sounds like your speakers are under water. If that’s the case then I just choose SB Pro in the sound setup and that never lets me down.

As I mentioned before it is apparent in Wolfenstein 3D which has only PC Speaker/Adblib/Sound Blaster support, it is even audible in Doom when using Sound Blaster mode. I wonder if the problem could be because of the ES968F which is present on this card and not on the E688 card. What is the purpose of this chip? The only thing I could find is that it provides some 3D audio capabilities but I'm not sure what is meant by that.

The ES968 chip used an algorithm from Desper to improve the spatial representation of sound from a stereo source. It's similar in application to the SRS circuit many high end TV sets had back in the day. If you can find a way, I suggest turning it off.

I have no idea if that's possible but I will try it. It seems that it's another use is to turn non PnP card (i.e. ES688 or ES1688) into PnP card.