This is one of the most common problem for all Sony smartphones. The phone speakers works fine without any problem (Ringtones and message sounds) But the internal speaker will stop working (While playing Video and Mp3 files). We can solve this by 3 simple steps. 

By restarting Smartphone : 
For some phones restarting would solve the problem. if it doesn't work try next step.

Disabling Sound Enhancers : 
Go to    settings > sound > uncheck the options Clear Phase and  xLOUD.

If this step doesn't work try the third one.


Installing SoundAbout app :
Install SoundAbout app from google-play. go to media audio > select speaker. (by default it would be "let the app decide") 

After 2 or 3 days change the option from speaker to "let the app decide"  This app will solve the problem eventually. Check this by disabling SoundAbout service after 1 month. if your phone is okay, then uninstall the app.