The good news is I’m not struggling with my target speed. I’m struggling with 10wpm faster.
The bad news is the time listed in the titles for at last some of the computer-speech files is wrong.
So, I strongly suggest you do a bit of math with the time of each file and the word count of the passage before you use it. Hopefully there will be a file close enough to your target speed for enough passages that it won’t slow you down.
At this point, I’d rather work on shorthand than programming. The files are still a useful progression of speeds, just not the standard every 10wpm.
If you need a specific speed, Audacity’s Change Tempo works well. Set the target length and it will adjust the speed while keeping a normal voice. You have to do it for each speed for each file, though. (A programmer might be able to automate it, but they’d have to learn Audacity’s scripting language first.) This method produces good enough results for speeds 70% to 130% of the original.
You can then listen to it immediately in Audacity or save it as a WAV file.
Express Scribe will take an mp3 file and play it at a percentage of the original. I think you set the percentage to Target Time / Actual Time, my brain is a bit off today. This method will not produce a file you can take with you.
If there’s enough interest, I can take another look at the program, but it will take several hours to remember how it work and find the problem.