Video sound

Hi, I am kinda new to WordPress, but I am following lessons through evening school. I just started my first website, using the free Rock Star theme. Up till now I am already happy with the result of my site, but there seems to be a big problem with one issue : all the videos don't give sound when being watched on an iPhone or on a Apple tablet. It gives no problems when I play it on my Windows laptop or on my Huawai smartphone. Any suggestions? I am a beginner, so I hope the problem won't be too technical ;-) My site : www.morphinemurphy.be Kind regards, Ricky