The mode you should select for CB communication depends on your needs and the distance between users. The main advantage of AM for CB is longer range, and the main advantage of FM is sound quality at close range. Having a dual mode AM/FM CB radio allows the user to modulate between the two types of channels to find and use the best signal for communication based on their needs.
For example, if the sound is coming through muffled while communicating locally on an AM channel, switching to FM mode on the same frequency can strengthen the signal and greatly improve the volume and clarity of the sound. But if you’re trying to communicate with other CB users at greater distances, AM frequencies will likely be more reliable.
Having a dual mode AM/FM CB radio allows a user the option of choosing a modulation mode best suited for communicating with another station. For example, if the sound is coming through muffled while listening on a channel using the AM mode, the other station may be operating in FM mode. Try switching to FM mode. If the sound becomes clear and intelligible then the correct modulation mode has been selected to communicate with that station. There may be a bit of trial and error while choosing which mode will best suit in any given circumstance.