If you have the source you can edit which songs they play. It doesn't just have to be the soundtrack to half life.
You can rip your favorite music off of youtube with sites like www.vidtomp3.com, from there use editing software (mabye audacity?) and convert the file to a lower size, i google searched and found a program that lets me change the hertz in a file so it sounds a bit less quality with a much shorter download. Also make sure you precache the sounds you add.
It has an .ini file where you type in what songs you want it to play.