1. download http://www.hlss.us/downloads/dMC-r11.5.zip
2. unzip it and install it
3. open that program and then choose the music u want to play in CS
4. Set frequency lower than 11050Hz <~~ thats really important as most of mp3 file is 4xxxx Hz...so lets say set frequency to 11025Hz.
5. create a file called "mp3" in C:\Program Files\Steam\steamapps\your email\counter-strike\cstrike
6. put the converted mp3 files to that C:\Program Files\Steam\steamapps\c443122@yahoo.com.hk\counter-strike\cstrike\mp3
7. in the game, open console and type " mp3 play mp3/1.mp3 " if the converted mp3 file is called 1.mp3
P.S. you can set the mp3 volume between 0 to 1 by typing "MP3Volume 0 ~ 1" in console and you can stop playing mp3 by typing "mp3 stop" in console