If you haven't cleaned out your speaker for a really long time you might want to try that, it could be filled with dust and ear wax. Or it is possible you spilled something onto your speaker which can distort the sound coming out. If the issue is still persistent or the issues I mentioned are not the issues you're having please check out the troubleshooting page: Toshiba Satellite C855D-S5303 Troubleshooting

There could be something covering your air vent, make sure your air vent is not being blocked. If there is nothing covering your air vent you might need to clean out your air vent, it could be filled with dust and dirt. If this was not much help to you please check the troubleshooting page: Toshiba Satellite C855D-S5303 Troubleshooting