Is it normal for babies to laugh without any sound?

Yes, I think that's possible. I only heard my baby express real laugh (with sound) when she's 4 months old and that happened very often that time. Usually she'll just laugh silently - you know it's laugh as her cheeks rise, shoulders bounce, dimples coming out haha! Then she started making sound laughs when she turned 6 mons and now she giggles whenever she finds smething funny and ticklish :) Sweet kid!