New subscribers only, ends today. In celebration of Prime Early Access Sale (Oct 11-12), Amazon is offering 4 Months of Amazon Music Unlimited Service for $0. After the 4 months, it's $8.99/mo. for Prime members and $9.99/mo. for non-Prime. 

YMMV: Some accounts may only see 3 free months. If you've already used AMU before, no offer will appear.