If you are having issues with sound when using the IFT app on iPhones please read on:

There are two options related to sound issue in iPhone
1) sound & 2) volume
iPhone user must enable sound to hear voice instead of doing volume up down. 
Please check if your mute button is ON (the physical button on the side of the phone or the Assistive touch). When this is ON, sound cannot be heard, so turn off the Mute button.

Here is how to fix the issue:

Step1: Click on assistive touchStep2: Go to Control Center

Step3: Find sound icon, and enable it if its disable.

Please also watch the attached video for troubleshooting the issue.

If the issue persists, please let us review and reply:
1-Does the issue happen with only downloaded videos or also non-downloaded videos?
2-Are you using ear/headphones? what kind are they? does the issue happen when using normal phone speakers?
3-What are your phone details (iPhone model and iOS version)

Please also check the Appstore for any updates to the IFT app and install that.