Welcome to Worldbuilding, <PERSON>! Do not be discouraged when contributors urge you to do your own research. You will find that frank criticism is far more useful than compliments and ego-stroking when you get down to the serious business of building worlds.
Now to your question. Is it possible? Yes, because you have provided your merfolk with lungs, but it is, in my opinion, cumbersome from an evolutionary standpoint. Why would a creature so well evolved to live underwater as a mer-person use vocalizations evolved for a gas environment in the marine environment?
In this case, I would suggest thinking about common evolutionary adaptations that are readily observable in the world and then expand on them in your own world. Think about the vocal sounds that terrestrial mammals make and why, and then think about the many different ways that marine life communicates without the needs to vibrate a set of vocal cords with a moving gas. Certainly, your merfolk could use something like a highly advanced system of sign and body language, and they probably should, but why stop there? Why not give them special adaptations, like skin with chromatophores (like squid and cuttlefish) with which they can communicate voicelessly through pattern and color? What about bioluminescence? Take some time to study how marine mammals (marine creatures with lungs) communicate underwater to get ideas, such as cetaceans. 350
In the process, don't forget mammals that spend a lot of time in the water, too, such as walrus, seals and otters. The natural world will give you ideas, and then you can layer on your choice of magic or specialized evolution to complete your organism.
Also, I would spend some time thinking about whether your merfolk started life as terrestrial and moved a marine environment, or vice versa. This will help to guide you in your biological designs. Do your merfolk ever come out of the water and converse with terrestrial creatures? If they do, must they speak in words? I understand that they have lungs and gills, so you have addressed the issue of oxygenating their tissues, but do they actually have vocal cords? If they do, this suggests that they were first a terrestrial people that moved to the marine world. If they lack vocal cords and instead have something like chromatophores in their flesh, then this suggests that they began as a marine people and made adaptations that encouraged symbiosis with terrestrial creatures.
Remember, <PERSON>, at all times the study of the natural world will help to inform your creature lab. Hope this helps! 671