Hi,
The loudspeaker may be clogged or faulty.
Here’s a [https://www.youtube.com/watch?v=KUd1a-CFry8|video] that shows how to replace the loudspeaker in the phone.
Here’s a link to some [https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=lg+g5+loudspeaker&_sacat=43304|suppliers] of the part. It is only shown to give you an idea of the cost of the part. There are other suppliers online that may suit you better. Just search for ''LG G5 loudspeaker'' to get results