For music, the only thing I could get to work correctly is using the flowgraph node

"Sound: PlaySoundEvent" (or similar)

Put the music sound file in the parameters (you need to have an mp2 [just rename an MP3 file to MP2], and a wav file in the same folder to get it to read it), and uncheck "Voice"

Then, put in the node "Game:LocalPlayer" and hook that into the red input entity port on the "PlaySoundEvent" node.

Then, just have an area trigger or something start playing it.

There is a problem with this method though. First of all, the music/sound will move between the left and right audio channel on the speakers depending on which direction a player strafes. So when the player strafes right, the music will be louder in the right ear than the left one. It is very subtle, but it's definatly annoying.

Also any reverb area's or special sound-changing area's will affect the music (so if you are in a reverbvolume area while the music plays, your music will be reverbed too).

However this is the ONLY way you can get custom music/dialog in the game without modding your game folder as far as I have seen.