Hi
Try to find something modern, perhaps something where the character is talking to a family member or someone they know well or choose something a bit introspective, a memory described for example. For a theatre audition piece it is often a good idea to look for something where the stakes are high and I might suggest avoiding introspection, domesticity etc, but if you're looking for something suitable for film it is a different task.
The reason you are finding it difficult (I suspect) is that monologues are very rare beasts on TV and film. On screen, we are "shown" the story, rather than "told" it. Characters on screen very rarely bare their souls and recount their experiences at length - we find out what they are thinking by how they act and react. Monologues on film tend to be either 1. Oratory (which wouldn't work well for an agency audition 2. A courtroom speech (also not ideal) 3. A massive rant either comic or serious(hardly low key!) 4. A voiceover (which needs you to see what it is "voicing over").
Occasionally you see a sort of "video diary" confessional sort of thing, but even that isn't ideal for audition purposes because you are doing the highly unusual thing of looking straight down the camera lens.
So.... sorry.... that's what not to do.
Have a look at Charlotte Jones, Amy Rosenthal (there are some nice bits in Henna Night). If you have a few plays to consider, then something that works really well for screen is cutting and pasting several very short speeches together, leaving out the other character's words (providing it makes some sort of sense without them). The reason this can work well is because it gives you a real "beat" to play between the short speeches where in your head you can react almost as if the other character HAD said what they say in the play. I don't mean leaving a pause and pretending the other character is there, but behaving as if your brain has suggested a new thought which you then respond to with a different energy - do you see what I mean? This keeps what you are doing "alive" throughout the speech.
I don't know whether that ramble is helpful...
