Your problem is likely due to some kind of damage to the speaker, which makes the sound muffled or distorted. You will need to replace the speaker.
If the speaker is replaced and you are sure that you have installed it correctly, but the sound is still low while volume is set to the maximum, your battery could be weak. Check out the monitor's [[Uniden Guardian G955 Troubleshooting|Troubleshooting Page]] for further help.