gradual tempo increase

It's certainly possible to do this to a track, as SuiteSpot points out - but not to an MP3 as it stands. You can't do anything much to those at all, and you certainly can't speed them up. If you open your MP3 in Audition, it will open as a decoded MP3, in other words, a wav file. And it will be this that you are increasing the tempo of - your original MP3 won't be touched unless you resave the file. And if you do that, there will be a quality drop, because it has to be re-encoded. My suggestion is that if you are going to do this, save the resultant MP3 at the highest rate you can, as this will minimise the additional artifacts that will almost certainly be introduced by altering the speed.

Check this thread out 

https://community.adobe.com/t5/audition/how-do-you-gradually-slow-down-a-track/m-p/5309486?page=1

You just need to do the reverse