Commonly, Audi nos is translated as "hear us". Audi is the imperative form of the verb but nos is ordinarily translated as "we". How does "we" become "us"? Is there some grammatical factor operating or is it merely some form of convention?
Nos can be either "we" or "us." The nominative and accusative for it are the same.
It's not that different from the English "you", which can be the subject (vos auditis, "you hear") and the direct object (audimus vos, "we hear you"). As you can see, the Latin plural pronoun vos can also be nominative or accusative.
The full paradigm of cases shows an even more about of forms of nos which are the same as other forms. Context will be key to figuring out which is which, much like the word "you" in English:
The key concept here is that of grammatical case. Latin has several cases, of which two are important here: the nominative and the accusative.
Let me illustrate this through English. In the sentence "We hear you" the subject 'we' is in the nominative case and the object 'you' is in the accusative case. If the roles are reversed, we get "You hear us" where 'you' is nominative and 'us' is accusative. English mostly marks these roles by word order, but some pronouns have visible cases.
In Latin most words have a visible case, but the pronoun nos ('we') looks exactly alike in the nominative and accusative — but not in any other case. That is, nos can mean either "we" or "us". Hints to the correct translation are given by context, word order, and most importantly the forms of the other words in the sentence.