Instructions
Load Steam and select the "Tools" menu. Choose "Source SDK" from the list of options.
Choose to load the map-editing program Hammer and then select the game type -- such as "Half Life 2" or "GMod" -- that you wish to associate with the map.
Load an existing map or start a new one and then select the object creator from the menu on the left. Select any object and place it on the map where you wish to play a sound.
Right-click the object and type "env_soundscape_triggerable" in the class field. Select "Soundscape" from the key-values list and type the type of music or sound file that you wish to play when a player is near the object. A list of sound files can be found in the drop-down menu on the right.
Press "Ctrl" and "T" simultaneously to open the Trigger Events menu and then select "Soundscape" from the drop-down menu. Select the object you created in Step 3 in the drop-down menu on the right; the sound file will trigger whenever a player nears the object.
Change the source of the sound by changing the object type or moving the object to a new location on the map.