Firstly we highly recommend studying using a tablet or a computer. A larger screen is a lot nicer to look at, a lot less strenuous on the eyes and also sometimes the writing can be too small to read on a mobile screen. However if you are happy to learn on a mobile then here are some tips for you.
Always watch in full-screen Landscape mode
Make sure after clicking play you turn your phone onto its side and make the video full screen. This will help to give you the largest possible video size for your phone screen.
Unfortunately phone speakers sometimes will not pick up all the sound of our videos. If you cannot hear the teacher it is highly likely that this is the reason why. To solve this, plug in headphones or speakers into your phone (Or even bluetooth speakers) and you will be able to hear everything.