Did you wet the phone at all? Or may it have got wet without you noticing?. If so, you could have water trapped in the speaker which is safe but you need to let it dry out. You could try blowing on the speaker to loosen the water particles and dry it that way, using a vacuum, using a hair dryer on the lowest setting directed at the speaker or the trusty, container of rice option. This was draw out any moisture in the speaker.
Is the sound from the speaker distorted in any way or simply low volume? If its low volume, I'd say water has entered it, if there is distortion something more hardware related is going on.
Is it just the main speaker or is the ear speaker being affected too?
