Samsung phone have sound issue , speaker voice are very low andits ahrd to troubleshoot this issue , to handle this issue you need to find exact issue of this problem .Samsung Galaxy will ring but speaker will not work or no sound at all for calls, apps, music etc or sometime phone will stuck in headphone mode sometime headset is causing the issue so disable headset setting restart phone and then try again.
To Handle this issue you need to take some steps try performing a factory data reset.
Why there Low Sound in Mobile Phone Speaker
If still issue is there then there is possible reason that your speaker is blocked with dust , you need to clear this dust to get clear voice.Due to sweating speaker slowly get jammed and it reduce the volume of your phone.So you need to clear that. Use phone cover to avoid this issue in future.