How To Fix Songs With Drifting Tempos
Most songs do not keep a constant tempo and it makes it very difficult to make a remix or mash-up.  So, what can you do about it?  With Mixcraft, you can dissect the song into multiple beat matched clips using a metronome.  Then you can mix down the modified song to a new WAV file.

Let's edit the song Not Ready To Make Nice by the Dixie Chicks to be at a constant tempo.  Its detected tempo was at 83.243515 bpm, but as you can see from the Loop Editor on the Sound Details Tab, the sound's tempo drifts from 81 bpm to 84 bpm.

Since the song starts at around 81 bpm, we adjust the song to 81 bpm initially.  Turn on the metronome and listen to the song.  When it is no longer in time with the metronome, you'll need to split and adjust.

In this case, the tempo speeds up in the middle of measure 19.  The actual beat (shown in red) should have lined up with measure 20.

If possible, pick a section of sound a measure or so before the obvious audio drift.  Make sure that snap is off on the Snap Settings so that you can make a precise split.  After the split, select the right hand side clip and adjust the tempo on the Sound Details tab.  

On the Loop Editor, you'll be able to see a dark gray area which shows that the left side of the sound has been trimmed by the split.  You can zoom in to the light gray side to visually determine the best tempo.

Proceed from the left to the right on the sound with the following process:

1. Listen to the song until the beats no longer sync with the metronome.
2. Split the active sound clip a measure or so before the drift is noticeable.
3. Assign a new tempo to the right most clip you just created from the split.
4. Repeat this process until you get to the end of the sound.

When you are done, you'll end up with a group of clips like this:

Mix it down to a new WAV file for future remixing!