What do you mean by "slow down music" ?

 

I use DirectX plugins with a sound editor like Sound Forge or Cubase. They allow you to slow down the tempo without altering the pitch, which is useful for creating sound effects or learning fast guitar riffs.

Here's some standalone software
Amazing Slow Downer www.ronimusic.com
Guitar & Drum Trainer www.topshareware.com/Guitar-and-Drum-Trainer-download-14624.htm
SlowGold www.softslist.com/download-1-50-13138.html

 

ACID Pro 5.0 is a really good program i use it a lot for my videos

 

Sound Forge gets my vote.