I am trying to play only part of a piece of dialog that is in a soundfile.
How can i make it so that it only plays it from halfway?

I tried setting the volume to 0 then when the right line comes by putting it to 10, but that didnt work.
it just played the song from the beginning...

Out a delay in the outputs. Say:

OnTrigger > PlaySound > 0.00
OnTrigger > StopSound > 11.00

Correct me if I am wrong.

yep that is if you want to stop a sound halfway
i want to start and stop sounds halfway...

with volume 1 its audible? maybe volume 0 completely turns it off, and you need to start it with 1 (with looong fadein), then make it 10 when it reaches that part (with a long fadein as well), then stopsound.
so startvolume 0, volume 1, fadein time 0:30, at 0:30 set volume 10... (if the dialog is at 0:30) maybe works, but I experienced weird things with sounds too, maybe its easier if you just cut that part out and add as a custom sound.

The volume input should work. If it's because it is set to 0, then try putting it as low as possible. e.g. 0.01

Bumping because I have this exact same problem. I must have tried everything, I tried checking/unchecking all of the flags, tried using different inputs. Volume/pitch always restarts the sound from the beginning. Fade ins don't work at all anyway, I even tried setting its source entity to something far away and then moving it to the player at the appropriate time, but that just made the whole thing play full volume everywhere. I don't get it, nothing makes sense.
It's a voice over file, if that makes any difference, and I really, really want to refrain from using custom a sound just for it.