@annasitumorang start off by cleaning the speaker grill. Those speaker openings are very small and a lot of debris collects in those. If that does not work, you may need to consider replacing the speaker.
Since iFixit does not yet have a guide for this. Use something [https://www.youtube.com/watch?v=aA5OXTgdgCI|like this] video to get to it. You do not have to mess with the main board. After the initial opening go to 2:12 to see how to remove the speaker assembly. Replacement speakers are readily available online. Just select a speaker that is for your exact model.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!