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I want to be able to end a music clip with the speed of it slowly decreasing and then make it fade out. And them do the exact opposite in beginning a music clip by slowly increasing the speed. How do I do this on Audacity 
-Jon64
If I'm understanding what you're trying to do correctly, Littlebrick actually posted a topic about this exact same effect less than a month ago. This was what he came up with. Insert patronizing quip about the search function here. 
I'm not sure in audacity, but you can do this easily in the free version of Goldwave, it's called "doppler" and lets you easily change the speed of a sound dynamically along a timeline.
In Audacity, you select the part of the music video you want to slow down (or increase) the tempo of. Then, while still selecting the part, you go to the Effects menu, and click Change Tempo. (It's important to use Change Tempo and not Change Speed, as with Change Tempo your music's pitch will stay the same.)
If you know how many BPM (Beats Per Minute) your ending or beginning is, and how many BPM you want it to be, fill the two in. If you have no idea about BPMs, just use the percentage. Slowing down can be done with negative percentages, like -10% (or any other number between 0 and 100, I suppose), and speeding it up with positive, like simply 10%. You can also change the number of seconds.
Be reminded however that if you do drastic speed changes (like -20% or +50%), the audio will become distorted. This is because the digital audio is essentially split up in very small points played right after each other. For example, a 44100Hz sound has 44100 of these "points" every second. Decreasing the speed is basically asking the software to make up new points, or copy other points; increasing the speed means deleting some of these points. (This is a quite simplified explanation, and not actually how it all works, but it illustrates the point a bit.) So don't go changing your tempoes too much! 
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