So many types of agents, so many possibilities... Which is the one you need? Is it a police agent, a secret agent, or just a 'simple' business agent? In any case, you need to look for your particular type of agent voices.
Agent is essentially any person who directly acts as a representative, or exerts some sort of official power. There, the key lies in two terms - an agent is the one acting and she/he is a representative of an institution or bussiness. Very often, agents have a very characteristic manner of speaking, particularly in the manner in which they formulate their sentences. Also, they tend to use more formal type s of expression, as if they are reading a pre-set text.
There are a few tiers of audiences where the use of agent voices can be quite suitable. One is the audience that needs to hear an official statement, particularly if it includes a set of instructions. There, the agent voices represent official institutions, police or army, or any similar institution. There are audiences that need to hear business information or instructions, like in business guides or manuals where agent voices can be quite suitable. And then, there are consumer audiences, where voices of sales agents could come in quite suitable.
The agent voices that represent official institutions and say, police essentially sound formal . Depending on the situation, they can also have a detached or stern, strict tone. On the other hand, business and sales agents have a more approachable, sometimes even friendly tone, as they need not just to transmitt information, but present their business or product to the customer in the most appealing manner.