Warner Music explicitly states that they do not accept song demo submissions.
Please read the disclaimer below to better understand their policies.

Demos should only be submitted to Warner Music Group’s record labels (e.g., Atlantic, Elektra, Warner Records, Parlophone) for consideration, not directly to Warner Music Group. However, please note that Warner Music Group’s record labels do not accept unsolicited materials. This means that your demo must be recommended to our labels by an established industry professional (manager, agent, lawyer, journalists, one of our artists, etc.). Industry professionals that understand the creative goals of our record labels are best suited to make these recommendations.

We strongly suggest you seek out the assistance of one of these individuals.