Hi @odesu ,
There are two microphones in the Note 5.
The 2nd one is a noise cancelling microphone but I believe that it is also used when in loudspeaker mode.
Here is a link to the ifixit Samsung Galaxy Note5 Teardown guide.
In Step.12 you will find the location of the noise cancelling microphone.
Here is a link to just one supplier of the part. It is not a recommendation to use them. It is shown to give an idea of the cost of the part. There are other suppliers online that may suit you better. Just search for galaxy note 5 noise cancelling microphone part to get results.
Before you go to replace the microphone or even to check that it has come loose etc, try starting the tablet in safe mode just in case, for whatever reason, a downloaded app may be the cause of the problem. If it works OK in safe mode then the trick is to find out which app is the culprit.
I don't know your phone but check at the top of the phone if there is a voice inlet hole (similar to what is on the bottom of the phone) and see if it is blocked. Use a strong light and a magnifying glass to check. If there is a voice inlet hole and it appears blocked (or even if you can't tell) use a vacuum cleaner to try and clear any obstruction. DO NOT use a probe to clear the hole as you may damage the microphone.