Greetings Sherrybaby44,
If the issue is related to this one video, you may want to check with your friend to verify that the video was sent with the sound included. Some apps and features allow for a video to be sent without sound.
If you have confirmed that the video was sent with sound, you may want to check to see if the issue is related to all videos or just the one. Have your friend send another video with sound to see if the issue persists. If so, have a different contact send you a video with sound. This will help isolate the issue to your friends device while recording, or your device.
If the issue seems to follow your device, try to Restart your iPhone - Apple Support. We know this sounds like a simple step, but sometimes a fresh restart can correct unusual behavior that pops up.
We hope these steps help to isolate your issue so you can find a solution.
Best wishes!