Q: The sound on my iPhone 5s when in video mode is very faint almost non existent

Greetings colinfromswansea,

Welcome to the Apple Support Communities!

I understand that the speaker volume on your iPhone 5s is not as loud as you would expect. To begin troubleshooting this situation, I would suggest reading over and working through the suggested steps located in the attached article below. 

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

Cheers