As I said, before you start with a dialogue/monologue piece, send me or show me in class 5 different clips that you found, don't just start with the animation.
Don't use any famous actors, especially comedians, like Robin Williams, Jim Carrey, etc. If you choose a sound clip that's well known (like anything from "Office Space"), then people will think about that movie and the actors that say the line, not the character that you are animating. They already have an image of how the character should act, based on the real life actor. You don't want your character being compared to the voice actor, you character needs to stand on its own.
So find something from an old movie, or just obscure movies, or actors that are not well known, something that hasn't been used before. I also wouldn't use a clip from an animated movie or tv show.
The clip should be in English. Unless the animation is super sweet, people will get put off by something they don't understand and no, subtitles won't make it better either. If you have been working in a different country, then that's another story, but now you have the choice, so choose wisely.
I'm writing this from the point of view if you're in school here in the US. I remember having audio tossed out by teachers because of foreign languages and hard to understand accents.
Obviously the animation should always count, but if you're applying at a company that does movies in English, and you have a choice in your sound clips, then I would choose an English one.
The clip can't have profanity in it, or anything that's racially or religiously demeaning. You're trying to find a job, so offending the people at the place you're applying is not the right move. Same goes for anything sexual. It might be funny to your friends, but the recruiter might think differently.
Either way, why lower the chances of getting a job? If you are established and you have a job, then go for it, it's your own stuff, but if you are putting the clips on a reel, then you have to cater to a specific audience. Every company is a different audience. Some can be grouped together based on the type of animation (for instance animated feature or animation in a live-action movie) but their style will still be different within the same group. Dreamworks, Blue Sky, Pixar, Sony, etc. they all have a different style, some more recognizable than others, but still. Now, if you send shots from "Robots" for instance to ILM, it might still get you a job, but you will have a bigger chance if your work reflects the type of work that ILM is doing. Same goes for WETA or Sony (live-action division). And it goes the other way around. If you send "Pirates 2" shots to Pixar, then they can see that you can do very subtle facial animation, keyframe, mocap and realistic animation. But is it what they are looking for? No. You might still get a job, but you'll have a better chance if you show them cartoony animation, keyframe only, no mocap, with a very solid understanding of performance, especially if the company is in crunchtime. Imagine you got two reels, one is all cartoony, one all realistic and you need people for Pirates 3, right now. Will you guess and hope that the cartoony animator will be able to deliver what the show needs, or will you go with the animator that already proved to you that he/she can do it? I think you get my point.
With that being said, you can be the best animator in the world, but if you are a jerk, then your chances of getting the job are small to none.
Alright, back to the actual topic, audio clip.
Use something that has some contrast in it. A clip in which someone screams for 10 seconds is not a good choice. If during the first 7 seconds he/she mumbles and THEN screams, that's better, because there is contrast, there is a change, the character is going somewhere with the performance.
You need to show that the character is thinking, a thought process must be visible. You can do that by an emotion change, which is a popular assignment to hand out. Present the character with a choice he/she has to make, face him/her with a conflict.
Audio clips, which have a few seconds of silence in it are great, because during that time you are not "stuck" with the dialogue/monologue, you can do your own thing.
Don't use any clip that has exposition in it. That means a character explaining something, like an evil plan or something like that. It's like reading a manual, there's not a lot of thinking involved, or conflict, or contrast.
Hope that helps!