Call speaker not working

The OnePlus 2 uses two different speakers. The ear speaker just puts audio to your ear, while the loudspeaker provides sound for videos.

You will first need to remove the motherboard.

Once you have pulled it out the ear speaker should be easily visible at the top on the frame of the phone. Simply remove with some tweezers and insert the replacement.

As for the WIFI issues, could be some corrosion. I would clean the motherboard with 99.9% Isopropyl alcohol to see if that solves the issue.