Fasttyping

I have been using Express Scribe successfully for a couple of years and love it. However, it recently started distorting the sound when I slow down the transcription. I used to have no problem at all hearing at the slower speed but now I have to listen at 100% to get the clarity, but I can't always keep up with 100%. Does anyone have a solution for this? If so I would really appreciate it. I have a ton of work to do and am in a bind with this.