Watching last Saturday's episode I had to keep stopping and rewinding it to figure out what he was saying. He kind of slurs his words together and a lot of the time I find it difficult to pick up on what he's saying especially when he's talking quickly.

Anyone else having trouble understanding him or is it just me?

I have had no problems understanding anything The Capaldi has said.

Nope.

The only bit was when he was asking for the stairs that seemed mumbled so I didn't know why Missy then said she wasn't a Dalek.

Nice touch of humour there.

But I agree the sound isn't great for Capaldi sometimes. He is actually pronounced but the editing is dodgy once the music and everything is added.

They need to tone his voice up over the background noise I would suggest. He has a lovely soft tone. Like his version of the Doctor, he sounds very Tom Baker at times, particularly in this last episode.

I wholeheartedly concur. Doctor Who is one of a handful of programmes that I need to turn the volume up quite substantially for, in order not to miss anything.

Me too, it has nothing to do with his voice, it's the sound editing, the background effects and music drown out the voices on DW.

Strangely enough I had to turn up the volume a lot for Life Story with David Attenborough last week for the same reason.

The loudness of the instrumental music in Who has been an issue since it first returned back in 2005. It does sometimes swamp the dialogue - although I note there have been far fewer complaints on this forum over the past few years.

It does not interfere with Matt or David as they don't have quite the same soft tone, but even in Doctor Who extra Capaldi can be heard quite clearly as his soft vocals are not drowned by the background sound. His annunciation isn't the problem, the editing of the sound is. I remember it was very loud during Catherine Tate's episode at Christmas but her character got away with shouting. Capaldi performance is more subltle than that. He uses his voice cleverly, a shame to kill it with the sound.

Pure speculation, but maybe sound effects overshadow the dialogue because the sound as a whole is designed to be heard on a 5.1 surround sound system?

No problems hearing/understanding him at all.
OP - have you checked your TV is on the correct sound setting?

I don't have a problem understanding anyone else.

I have a problem understanding him sometimes and I'm Scottish! I often wonder what overseas viewers make of his accent.

it's delightful

His diction is good.
(I am British / English.)

I have terrible problems understanding him.
I miss much of what he says.
He does seem to slur his words but I also think the background soundtrack is too loud.

Sorry, what did you say?