You may be working with illegals...
Oh my goodness--when I read this, I had this image of you leaning in to stage whisper "You might be working with space aliens!" while they rip off their human-face masks behind OP.
U.S. urban centers, especially coastal ones, and even more especially that most Miami, run largely on an illegal workforce. And since these workers also need to shop, eat, and generally live as well, some knowledge of Spanish is pretty vital to living in Miami.
Legal or otherwise, many people who immigrate here choose not to learn English (or not to use it), often because they fear it will diminish their sense of their own cultural identity. Some worry that their children will not bother to learn their language at all, some don't plan to stay longer than they have to, some feel they are too old to learn, and some live in entirely non-English-speaking communities and never have any real reason to learn.
To OP: if people are really laughing at
you because you can't communicate easily, then they're not really your friends anyway.
But seriously: often people laugh to ease tension, but it's hard to feel on the outside of a joke--especially when you're not sure it's a nice one. I don't see much you can do except be as friendly as possible, brush up your Spanish as best you can, and, as you mentioned, look for another job if it's too uncomfortable.