First of all, it's "idle", not "idel".
Second, make sure your script is enabled as an Add-On.
Start a server.
Open the console, and type "findLocalClient().Player.playAudio(0,musicIdleSound);" without the quotes and press enter.
If the audio doesn't start playing (but you can still hear music from music bricks after making one), either
A: Your sound file is in the wrong place.
B: Your sound file has 2 or more channels (stereo / surround) instead of a single channel (mono).
Personally, I'd guess you goofed on B.