Hi,
The issue can come from a high temperature under the speaker (CPU, GPU, battery charger input, etc.). This cause the treeble or the lowd diaphragm of the speaker (circular membrane) slightly melt away, soften and doesn't work properly anymore. Then the speaker cannot transmit correctly the sound. The solution is to replace the speaker (not very hard, tuto can be found on the internet, only need Philips screw driver) and to find why the temperature increases a lot. Sometimes fans must also be replaced if they show signs of ageing or simply cleaned by removing dust inside.
Hope it will help.