ok on GOLDWAVE gte ur MP3 or watever and then open it. Click on file then BATCH PROCESSING then click on COVERT and set save as type: to WAV and underneath make it PCM unsigned 8bit,mono tick both the boxes and make it 11025 Hz. Now click on folder and find a folder when u want the final piece to go.
Now uve done all this u can use it in half life
i hope this was useful for u even though im NOOB at map making and have just started. Im still on my first map btw
If there are any Matrix fans then my map u will enjoy