What is meant by difficult audio quality?

"The characteristics of a difficult audio file include background noise, static on recordings, lots of voice overlapping, faint audio recordings, nonnative English speakers, very fast speech.

If you have one or more of these present in your audio, it is pretty much guaranteed to be difficult audio. Difficult audio files can be transcribed, but it takes double or more the effort and time to transcribe them.
We will be contacting you via phone/ email to inform you if any of the files are falling under difficult audio category"