Could be you blew a speaker. Often devices like these are best not used for long at maximum volume.
In the old days you could test speakers by using your device as a recorder (i.e. microphone) and see if the sound is detected. One simple experiment you can run is to plug your headphones into a microphone input, and speak into them. Fundamentally, both emitting sound and capturing sound is about dealing with vibrations in the air. The processes are opposite (one takes electrical signals as input, and outputs air vibrations, one takes vibrations in the air and transforms them into eletrical impulses).
Otherwise you might try and check for debris or dirt that clogged everything up or a wire that is starting to become loose or gives distortion, thus giving a weaker signal. iPhone 4(s) often had the debris problem due to the mesh getting clogged up with dirt or sweat, especially after getting wet by rain and others.
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