I don't know if it's an age thing or the sound production values being rubbish on so many programmes, I often wish there were subtitles for programmes filmed in English ( I guess there's a function on the TV I could turn on). I had another go at The Witcher these past couple of weeks and I swear I missed half of Henry's mumbling dialogue.
This may not be an age thing. We just finished season 2 of The Witcher. I’m under age 40 and had the same problem - there were a lot of lines that I couldn’t make out because Geralt sounded like he was mumbling. I mentioned it to my husband, and he agreed - so it’s not just you.
Overall I liked this season, but I found season 1 more compelling. Season 2 also could have used a lot more Geralt shirtless scenes, but that’s just a personal preference