The song your playlist is serving up *exactly* when you reach the field -- no cherrypicking.
As above, but you can fast-forward/rewind if that specific song sucks/is inappropriate.
A song your playlist served up at any time while you were writing the post.
A song you've deliberately added into your playlist so you can be listening to it when you fill the field out.
A (thematically appropriate) song you own, but didn't actually listen to while writing the post.
A (thematically appropriate) song you don't even have a copy of.
A song that doesn't actually exist, but would be thematically appropriate if it did.
Some smartass non-song to emphasize you have no music on (e.g.: "The Street - 'Traffic Noises' ").
Completely non-musical content.
A Sufjan Stevens song listed below in the comments, for the sole purpose of trying to break the field's length limit.