Internal microphone not detecting sound

I have a Macbook Pro early 2011 running Sierra. My internal microphone is not working. Internal microphone is selected in Mac preferences/Sound, it is turned up. However there is no input level when I tried it. But it does work when I use my iPhone earbuds with microphone when I tested it on Skype (but still no input level showing). I did recently have it in the shop because the I O U keys stopped working, they had to replace the top case where keyboard is located and give me a new sim card.

Anyone else or ideas, suggestions or fixes?