the XA1 Plus-model has a total of two microphones. A main microphone located at the bottom of the display used in calls and a secondary microphone used for other recordings such as video and calls on loudspeaker. To see where these microphones are located, please see item 4 and item 11 theoverviewof XA1 plus.
First of all I would suggest that you check if there is water, dust or any kind of protection such as a protective case or a protective film blocking the microphone or the area around the microphone.
I would also suggest that you use the Tests feature in the Support app to carry out a quick test to check that the loudspeaker is working properly. In order to do this, fins and tap Settings > Support > Xperia tests > Media.
Another thing that you can try is to reboot your phone in Safe mode. All apps downloaded from Play store is deactivated in safe mode and by doing this you can see if there is a downloaded app that is causing this. For more information and instructions on how to enter safe mode, please clickhere.
If none of these measures work to improve the sound from the microphone I would suggest that you try to perform asoftware repair using a computer and Xperia Companion. Please note though that a software repair will erase all user data, so remember to do abackup of your user data first.