Any of the points mentioned above will be reflected in the pay (or at least SHOULD be) and will be considered as challenging audio. Harder files pay more than a single person giving a speech in a studio recording setting. So do not reject an assignment right away because it's hard. Remember, they pay more!

Here are some tips to help you with hard files:

Don't give up. Don't reject a file 'cause it's hard. Sometimes you'll get a file, and your first thought will be, "Oh no, this is too much for me, can't do it" But most times, as you listen to the file, your ears will get accustomed to it, and you'll be like "Oh, yes, I actually can understand this speaker"

For hard areas, try using another transcription software. Sometimes using regular VLC software may sound clearer.

Feeling frustrated? Take a break. Go stretch, or better yet, stretch outside so you get air in your brain. When you come back with fresher ears, you'll understand more than before.

Go back to the hard areas after you've finished the file; you'll know more about the file (context, terminology, etc.) when you're done and feel more familiar with the audio.