Thanks for reaching out to the Apple Support Communities.
I understand that when you try to watch a video on Snapchat or Facebook it will play for a few seconds and then stop. You're also not hearing the lock sound when you lock your iPhone.
For the issue with sound on these third-party apps, I suggest going through the troubleshooting steps in this support article: If an app unexpectedly quits, stops responding, or won’t open .
Then, make sure the Ring/Silent switch on your iPhone isn't enabled when locking your phone. If enabled, you won't hear the lock sound.
About the buttons and switches on your iPhone, iPad, or iPod touch
Let me know if you have any questions.