The sound in the LG G6 sometimes fails to work where a user cannot adjust the volume while they cannot hear their callers or the callers complain that they cannot hear them well, yet they cannot increase decrease the volume on the LG G6.
This does not only apply to the call volume only but also in the music sound or audio. Below are some of the solutions to An LG G6 volume problem. If they do not help in solving the problem then you are advised to contact your dealer or present the LG G6 personally.
Switch off the LG G6 then remove the sim and insert it again
The LG G6 may have accumulated some dust in the microphone it is better to clean to get rid of it if there was something of the sort.
Switch off the bluetooth in the LG G6 since likely this could be the cause.
If the above does not work, put the LG G6 into recovery mode and also you can try to solve the problem by wiping the cache of the LG G6. Find here how to wipe the LG G6 cache.