Let's take non-audible off the table for now, because that opens up a whole other line.
To start I am not going to concentrate on how they might talk given those limitations, We should look, instead, at sound underwater. How it travels, how it's different than above the water.
You, as a human, hear things differently underwater because your system of hearing is developed for sound waves traveling in the air.
Take a look at this which is all about why that's so. Jist is basically:
Sound waves actually travel five times faster in water than in air. Underwater those sound waves don’t vibrate the ossicles bones in your inner ear. They go straight to the skull bones, vibrating that heavy bone you can touch just behind your ear...Because of that, you can hear higher frequencies underwater. You lose lower ones. And you can’t figure out the direction sounds are coming from.
So, look at an air-breathing underwater dweller that both talks and listens above waves and in the water. The dolphin.
They have two different mechanisms for listening, their jaw and skull for underwater, and their ear holes for above. Both ways can be used above and below the waves, but each is better suited for one way.
They also "talk" both above and below the waves. Their level of communication is debatable.
But certainly, you can use dolphins as a template, with range, types of sound, and methods of communication.
You specifically asked:
would be possible to have them be able to speak underwater in an audible manner or if it would all have to be non-verbal signals.
And the answer is yes, it absolutely can be audible, up to a point--but you don't say to who it should be audible. Dolphins hear differently than we do, and while we can hear SOME of it, and they certainly hear all of it, a human listening in would likely only understand what they kept in the range of human hearing. So audible to the dolphin folk (which is what I am calling them) but not always audible to a land-dweller like a human especially if dolphin folks do not want to be heard. We have a hertz range that differs. They just have to change the frequency.
Clicks, echolocation, chirps, all these can be put together into a sort of language.
But if you are looking at how "human" they might sound, it's going to be imitative at best, I think, on a more sophisticated level than underwater and above-water animals have done trying to mimic human speech. For that, look at this article, which is all about that and includes orcas that sound like us.
To my thinking, if an animal with these above/below water limitations can make themselves understood by humans, an intelligent version could certainly adjust their speech, especially for trade and such to be understood and to understand.
I would be having a heck of a lot of fun with this, because they will have a line of communication with their fellow fish-folk that would not be able to be possibly even registered/heard by land-folk.
You also asked this:
Would I have to make their hearing enhances as well? Thanks :)
That depends on what you mean by enhanced. If you mean that they would have a range different than human, then yes. But from a dolphin's point of view we are practically deaf, both in and out of the water. Bottlenose dolphins have a range of 75 Hertz to more than 150,000 Hz — well beyond the range of human hearing of 20-20,000 Hertz.
Hope that gives you a push in the right direction as to what questions you might ask the google,I know sometimes it's hard to even know where to start. Welcome to the site!