The most advanced race in my custom universe is a race of guinea pig-like aliens. They are short and have titanium bones, considering their homeworld has a gravity of 1.7 g. The nervous system features four brains which allow them to engage in a form of intellectual multitasking and they have four ears, one pair can sense infrasounds since they are a prey species whose predators use infrasounds for communication. I want their language grammar to look alien, but not too hard for humans in order to learn it and the language to sound weird, but very pleasant to the human ear. What are my grammatically and phonologically constraints when creating a language for this race?
Honestly you have no constraints
Except for the limitation based by their lingual physiology. The guinea pig is physically incapable of forming English words. Their language could easily be a logical order of the squeaks they make or with their intellect they could make translators to speak whatever language they want.