(AS3) sound wont close

programming flash
1

im trying to close a sound so that i can load and play another but when i call the .close() on my sound object i get this error…

Error: Error #2029: This URLStream object does not have a stream opened.
at flash.media::Sound/close()

it tells me this even though the sound object is currently playing.
here is the code im using to inistialize

mainMusic = new Sound();
			mainMusic.load(new URLRequest("lvl1.mp3"));

then elsewhere i call…

mainMusic.close();
				mainMusic.load(new URLRequest("WaterWorld.mp3"));

any ideas on why it wont let me do this?