Hi Tharinda,
I am Dave, I will help you with this.
1
Open the Settings App, then go to Update and Security - Troubleshoot, then click 'Show Additional Troubleshooters'
Run the Recording Audio Troubleshooter for there, then test if your microphone is working.
2
If that does not solve the problem, go to the support page for your laptop on the Acer website, then from there, download and install the Realtek Audio device drivers they recommend