I have fixed many problems like this. Most of the time I see that the anti dust mesh screen covering the speaker is clogged and caked with ear wax, makeup, skin oil, etc. For a thorough fix removing the screen and holding the mesh screen close to a light source should have abundant light. If light is barely visible, washing with alcohol and blowing from the inside out with compressed air seems to be the best fix. If concerned about opening the device then I would recommend alcohol and a fine brush similar to a clean short haired makeup brush seems to be effective although not as long term of a fix, it seems to do the trick for awhile.