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Who knew there’d be so much math?
I don’t think I’m alone using an audio editor like TwistedWave or Hokusai to determine/change the tempo of a downloaded audio clip. At the start of the conversion I’m pretty sure you’ll always have to count the number of beats in a clip, either by looking at the audio shape or just listening to it. That probably will never be automatable, but after that you have what you need to both determine the tempo of a clip and to change it. E.g. 60 / (AudioLength / Beats) will give you the tempo of the incoming audio, and (at least for TwistedWave) 100 * (NewTempo / OldTempo) will give you a number you can plug into speed change.
You have to use a calculator to do the math, but wouldn’t it be cool if there was a “Tempo” modifier in these apps, that given #beats and new tempo, it would do the math for you?
Who knows, maybe some app already exists. Anyone know of something like that? Obviously you could use timestretching in any app that has it, as long as it left the pitch alone (or at least gave you the option), but if it’s not in the app you’re working in, you’d still have to export/import.
There’s nothing to keep something like TwistedWave or Hokusai from adding it to their list of functions except the willingness of the devs to add it. Are you listening, devs? 
Comments
There’s this one - that may do what you want. Or it may just do a bunch of stuff you didn’t even know you wanted
https://itunes.apple.com/gb/app/musicmath-touch/id381525762?mt=8
There might be something in there! Laurent Colson’s An excellent developer. If something useful is missing, he might consider adding some features. I notice the app isn’t dead because he’s updated it a few times in the last year. Thanks for pointing it out to me.