Even with a humidifier I struggled for a long time with a dry mouth and sore throat. After having the info downloaded from my CPAP provider, I was told that I was having a great deal of leaking. The tech determined it was likely because my mouth was opening up and the air I was receiving through my nose was circling back out my mouth. He suggested chinstrap which has not only helped with the leaking, but the dry mouth as well. I'd suggest it to anyone using a nasal CPAP mask.
When using a CPAP, sore throat considerations might include being sure that you are not breathing through an open mouth, that your humidifier is operating with the correct setting and you have filled the reservoir with water, and that the reservoir and tubing are clean and germ free.
When using a CPAP machine with a setting of 10 or higher it is recommended to use a humidifier which also has a heating option to keep air from become too dry and causing a dry sore throat in morning. The small water tank can use tap water and filled nightly. The humidifier must be a part of the CPAP machine. External humidifiers will not help. From my experience with the humidifier CPAP machine there will be some moisture (water) build up in the airway hose. I found that this can be resolved by elevating the hose as much as practical immediately next to the machine. Since the moisture is heavier than air nearly all the excessive moisture will be returned to the machine. Not elevating the hose will result in bubbling sounds as the water build up blocks the air passage. Filing the tank all the way to the "line" is too much and can push water into the hose. Never fill the tank all the way to the line.