The speaker on my phone has bad quality
I can make and receive calls but the sound is distorted and difficult to hear
If this distorted audio is only coming from one of the handsets, it sounds like you could be dealing with a faulty speaker. This is usually the cause of any audio distortions you come across in any electronic device. Please check the troubleshooting guide for distorted audio to confirm that this is the problem. If this is happening to multiple handsets, however, you may be dealing with some sort of wireless interference, which would require some deeper digging.
Assuming it is indeed the speaker, though, you'll need to open the handset up and replace the speaker. At the time of writing this, it doesn't look like the repair guide for this component has been written. However, I would check back fairly soon to see if it's up. For the time being, if you can deal with only using two of your handsets, I would leave the handset that is outputting distorted audio off of the docking station and not use it at all. Once the repair guide is up, you'll be able to fix this issue yourself.
Hope this helps!
Loud volume can come out of a Panasonic phone in a funny way, you can easily connect a public address speaker. This works well at pool cabanas and in factories. You can get one at Radio Shack for about $40.You'll be surprised how far voices will carry with no additional amplification.